Sunday, May 3, 2020

Can Bioterrorism End Alien Civilizations?

'Terrorism' is used here to refer to small-scale groups attempting to achieve some political ends through the use of terror attacks, which are attacks designed not necessarily to cause great destruction, but to induce terror in a significant part of the population of a target region, which will then bow to the political demands of the terrorist group. Technological determinism says that technology dominates social change, and it may also dominate terrorism, one facet of a civilization.

In the early eras of technology, where knives and poisons were the only available weapons, assassination was the only type of terrorism that could occur. Directed against leading members of the alien civilization's government or economic structure, a terrorist group could hope that concessions might be made to their cause if the leadership felt unable to protect themselves. Infiltration of the ranks of those with guardian capability might be one of the social tools such a group might use, and suicide attacks might inspire the terror they needed to accomplish their ends. 

The invention of controlled combustion might lead to projectile weapons, but these simply make assassination easier. Bombs, however, open up a new avenue for terrorism, and that is attacks on infrastructure or on the public themselves. These weapons have the most effect in crowded places, and the obvious countermeasure is control of those entering these places, with some sort of measures designed to detect such explosive packages, along with the ability to carefully search the areas, arenas or whatever places a particular alien civilization likes to attend in large numbers, to eliminate such weapons from being installed and hidden prior to the crowd's arrival, for places with sporadic use. Continuously used places would have continuous checking in place or lockdowns during non-used times of the day. 

The advent to nuclear technology, in the middle of the industrial era, does not change much for terrorism. Nuclear weapons are very difficult to design and assemble, requiring specialists of many varieties, and terrorist groups are unlikely to be able to obtain such a quorum. They also require multiple unique materials, some very difficult to make from other, more easily available ones. Since nuclear weapons contaminate great areas of any planet where they are used, all regions on any exo-planet with an advanced alien civilization would be motivated to cooperate in restricting access to these end-materials. The costs of a nuclear weapon program are great, and if terrorism is something small groups would use, they would neither have such resources nor be able to deploy them, if they found a donor. The weapons are also large and hard to move and hide, and they give off telltale radiation, which can serve as another means of detection. Thus, the advent of nuclear technology into the collection of useful technology does not make terrorism any more powerful or easy to apply, just the opposite.

The beginnings of biology, specifically the biology of infectious organisms, may be a different story. The ability to capture an existing infectious organism, and mutate it, requires little money or expertise. Even a single talented individual alien might do this as the technology is not complicated to understand or utilize, once society gets some basic knowledge into its storehouse of scientific understandings. Recall that psychology and neurology come later on, so that the ability of the society to detect some mentally disturbed alien, having such a capability, is limited. This means that an alien society in this particular phase of its industrial era can be victimized by individuals or small groups who concentrate on contagious organisms. 

This capability exists even below the level of a terrorist group. Curiosity or some sociopathic desires could motive individual aliens to explore what they could do in this area, as there may not be any knowledge yet about how to train young aliens to prevent their involving themselves and others in dangerous activities when they grow older and more informed and educated. Neither would politics be a solved science by this time, so there may be personal or political disputes that could motivate such talented individuals.  They might develop some organism, protect themselves and those they care about, and release it to see what happens. If it was based on an infectious organisms, the mutated version might be contagious as well. 

If amateur biologists can create mutated viruses, what could a terrorist group do? They might be able to operate in two stages, one: where they try all types of viruses in different locations to see which ones might serve as a terror weapon, and two:, bioweapon where they induce some cases of their chosen infectious organism into some locale that they have access to. 

A bioweapon attack, even on a small scale such as a terrorist group could manage, requires social controls to be put in place, rapidly and severely, if the contagion is to be controlled at a very low level. Those regions which can do this might be relatively immune to bioterrorism, but those which are not, for any of several reasons, could be held at risk by a bioterrorist group. After one or several bioterrorism attacks, it might be clear to all regions that they need to prepare themselves against such attacks. One way might be to scour the whole exo-planet for biology laboratories that bioterrorists might exploit, but since they can be quite small and do not need exotic unique materials, finding them all might be difficult. The other way, if the region has the resources and the governmental excellence to do this, is to organize a reaction to any attempts at bioterrorism, all the while reducing the locales at which it could be done. 

If these countermeasures against bioterrorism, in attacks or in threats of attacks, are quite expensive to a region, it might try to negotiate its way out of them with one or more bioterrorist groups, but since they can form easily, this might not be a long-term solution, and the expensive countermeasures are the only solution. If the costs are so large that the alien civilization suffers a reduction in affluence, in living standards, and in the means of survival, then perhaps the civilization will begin a slow collapse. 

The other solution that might be taken is technological suicide, where the alien civilization as a whole seeks to ban biological knowledge from being gathered, collected, or disseminated. This means that asymptotic technology will never be reached, the ability to diffuse bioterrorism will never be accomplished, and the civilization will go into stasis and collapse. A solution near to that is to strongly limit the knowledge of biology to tiny numbers of aliens, in the hope that this knowledge will not diffuse out to potential bioterrorist groups. This would seem to be a more rational solution, as it allows work on automatic generation of antidotes and antigens to continue. Thus, bioterrorism might certainly slow down the progress of an alien civilization, but it is unlikely to destroy it, and would therefore not be the method by which aliens are prevented from reaching Earth.

Biowarfare and Alien Civilizations

Warfare has been so common through the last several millenia on Earth that it might be thought to be inevitable that it would occur on all exo-planets with thriving alien civilizations. The killing of other individual aliens and the destruction of their property, on a large scale, can be motivated in many ways. It might be the equivalent of envy, hatred, greed, love of destruction, desire for power, wishing to spread one's world-view or religion, fear, and likely others. Since there are so many reasons for having a war, wouldn't there necessarily be some?

The antecedents of mankind's love for war might be their evolution as hunters. Killing large game and killing other aliens is not so much of a jump in direction as eating only fruits and vegetables and then starting to kill other aliens. Can only omnivores evolve intelligence and eventually a civilization, or could herbivorous creatures do so as well? 

Perhaps this question should be asked in a reverse manner. Can herbivores who develop tree-climbing ability and then grasping appendages stay herbivores, or would the ability to reach nests start them on the path to eating eggs, newborn animals, young animals, and lastly full-grown animals? Raiding nests on the ground might start them off on the same evolutionary track. Given the nutrient value of eggs and young animals, this track provides significant advantages, and therefore it is likely that such creatures would not stay herbivores, but would evolve, step-by-step, into hunting animals, and then into tool-using hunters. This is the likely step before killing one another, and then as groups form, so does the concept of warfare. Warfare is therefore likely in the history of most alien civilizations in the galaxy.

Technological determinism says that society is shaped by the level of technology it has achieved. Warfare, as one feature of society, is also determined by technology, and as technology travels from stone and wood tools, to metals of ever increasing strength-to-weight ratio, to combustion in various forms, and onward to machinery, so do the tools of war. In the later stage of the industrial era, on planets with uranium in the ground not already decayed into too much U-238, nuclear weapons should be invented, and then the society would quickly realize the disutility of weapons of so much destructive power and requiring so much expertise to use. 

There is likely an overlap between the genetic era, when biology is being understood in many of its details, a precursor to genetic technology, and the last stages of the industrial era, that of electronics, automation and robotics. Breeding of plants and animals would have been proceeding for the whole age of the society, using trial-and-error techniques, and as the understanding of disease becomes widespread, the concept of bioweapons does also. One can use trial-and-error methods to breed disease organisms as well as socially useful organisms. Initially, the analogous use of bioweapons would be tried, similar to chemical weapons, such as by explosive canisters or sprays, applied on the front lines of armies, but these methods have quickly-discovered drawbacks of self-contamination and countermeasues, such as personal protective equipment.

Contagion is a more appropriate use of spreading a bioweaponized virulent organism. If one region has a particular and unique type of crop, which provides a substantial fraction of the nutrition for this region, then an enemy region could attempt to devise infectious organisms which would spread widely through the crop, eliminating its value. If the crop was annual, the yield would plummet. If it was a perennial, the productive plants would fail to grow the product, or even die. No such type of attack would work if all regions grew the same range of crops, however. Analogous arguments would work for animal husbandry as well.

If there was some unique genetic characteristic that most of the inhabitants of one region possessed, and it were possible to breed an infectious organism that would only attack those inhabitants with the particular feature in their genes, an analogous attack could be made. However, if this genetic dissimilarity is not wide-spread, or no organisms can be made to focus on one that exists, biowarfare can only be accomplished through a more organized and insidious means. If contagion is the means by which the infectious organism spreads, then the attacking society must somehow have some characteristics that allow it to be only slightly affected, which the opponent must have the opposite characteristics. If the disease is mediated by insects which live in unhygienic environments, a hygienic region could attack a unhygienic one. The reverse is obviously not true, but if there is any infection-carrying options, such as pets of some particular type, these might serve as the vectors for the disease contagion. 

If the disease spreads only from dead bodies of victims, then burial details might make one region more susceptible to being the target of a biowarfare attack. However, this is something that could quickly be recognized and altered, so such an attack is problematic at best.

If the disease spreads only through direct sharing of bodily fluids, such as blood to blood, it is not likely that it could be transformed into a bioweapon. There might be the equivalent of Earth's mosquitos on some particular exoplanet, but insect control is not difficult in an industrial civilization. Thus these diseases also would not serve well as bioweapon candiates. But if the disease could spread through indirect sharing of bodily fluids, or even without bodily fluids being used on the whole transmission path, then there might be a possibility of a bioweapon. If the infectious organism can spread through touch, or live on any kind of common surface for a period of time long enough for mutiple aliens to touch it, or travel on dust particles or water micro-bubbles, then the disease could act to have a large degree of contagion. 

If the attacking region has a way to prevent such sharing because of social customs or other social controls, and the target region has different customs or no ability to install social controls, then the opportunity for a bioweapon war might be possible. It would not look like any other type of war, as there would be no battleground or front lines, no armies involved in mass attacks, no industrial war machines being used, and perhaps even no declaration of war. The only thing that would happen would be one region would succumb to a high level of fatality, while another would not. Then economics would finish off the struggle between these two regions.

Could one or more biowarfare wars doom an alien civilization to collapse and never reaching star travel? This is not likely to happen, as social controls can defeat a bioweapon attack or serve as a protection of an attacker, so society might have some economic disruption during the period of the attack, but the attacker would not lose their grip on technology, nor suffer a great deal of economic disruption, and would be able to control the other region or regions and continue to pursue technology and eventually get to asymptotic technology. After this point, infectious organisms are easily controlled and no biowarfare would make sense, as antidotes and antigens could easily be generated as soon as the infection was noticed.

Recovery from Epidemics in Alien Civilizations

If an epidemic sweeps through an alien civilization, reaching all corners of the planet, and is lethal to some percentage of the population, the main drivers of the civilization's progress are not affected. Population count is not a direct cause of technology progress, and it will continue after some delay caused by the epidemic. What is important to maintaining the progress is having a quorum of intelligent, problem-solving individuals who can organize their work to push the envelope of science forward, and then to apply it to the productive activities of the population. If there is some fraction of deaths, maybe even as high as 90%, this does not mean that the genetic resources that are needed to produce the future generations of scientists and engineers are lost, it means the numbers are reduced and progress will be slowed down, or even degraded for a period of time. But it does not mean a permanent halt, and the timeline for this society to be able to make star travel work might be delayed for a few generations. 

To kill off the civilization as far as permanently eliminating their future progress, there would have to be a lethality level near to 100%, enough to eliminate so many of the population that there was a genetic reduction in intelligence. Is such a lethality level possible? Something lower than that might render the civilization incapable of maintaining its living standards, or even to preserve the existing level of technological know-how, but physical records and memories passed on to young aliens would lead them back to the standards they once had, and allow a resumption of the progress toward star travel.

Can an epidemic kill 100% of a population? This means that the contagion spreads world-wide, and that takes time, during which awareness of what is happening would travel all over the planet. Some response would be made, and the figure would drop below 100%. In the industrial era, when epidemics are possible because there is world-wide transportation and not yet rapid genetic developments of antidotes and antigens, there are still recourses to reduce the impact of the epidemic. Furthermore, with a wide mix of genetics for the immune systems, optimality having not yet been accomplished or even understood, there might be some aliens who are naturally immune, constituting some fraction of the population. And there might also be some individuals who are mostly resistant to the infectious organism's effects, recoving from it in more or less unimpaired condition. So, the achievement of 100% or very close to it lethality is unreasonable to suppose.

Before assuring ourselves of the recovery capability of a generic alien civilization, we might ask if there are any circumstances in which such a recovery might not happen. Regrowth needs resources, and if the civilization has already harvested the easy to gather ones, the minerals near the surface for example, could there be a barrier set up so that the civilization is bound down to a lower level of technology, one not capable of difficult extraction situations for critical items? Technological progess and resource development go hand in hand, and if the latter is impaired by what happened before the epidemic, could there be a strong barrier, sufficient so that the civilization would remain at some level, industrial or agricultural, forever? 

This is a question related to the particular planet upon which the civilization resides. Does the planet have large, relative to the usage rate of the population, amounts of most necessary minerals and energy resources? Or is the planet, owing to where it developed and the history of supernova generation of heavier elements in the clouds nearby, rather short of resources? If the latter instance, could the near exhaustion of resources in the industrial era could leave the surviving civilization with only too-hard-to-obtain resources remaining? This means that, during this alien civilization's industrial era, no one noticed, or if it was noticed, no one responded to the problem, and the resources available to the civilization were rapidly diminishing and growing harder to locate and recover, and instead of the obvious solution toward reducing usage with a world-wide reuse plan, they simply continued to work toward an early resource exhaustion. 

This does not make sense to rational people, but could there be some economic system which drove resource exhaustion heedlessly and recklessly. Could such an economic system stay in place when the costs of resources mounted steadily and significantly? This is an excellent question about the unbreakability of some economic systems. Can they be so firmly embedded in the culture that they would be blindly followed to near-term self-destruction of the civilization? Economic systems are in place because those who have the power to determine the ones to be used benefit from them, and so this question is, could these leaders of an alien civilization be only concerned with their own short-term benefits, and dismissive of what will happen to the civilization as a whole in only a few generations? 

This question takes us further afield. Recall that the science of training children, which involves setting goals for them in the deep subconscious, may be completely unknown to the civilization, and child-training and goal-setting left to random choices by those responsible for that training. Thus, short-sighted goals might be preserved, generation to generation, including the goals that those who become leaders have. This particular realm of science is likely only able to arise in the later part of the industrial era, that of electonics and automation, or even in the early part of the genetics era. 

There may be other mechanisms by which an epidemic could put an end to the future of an alien civilization, barring them from space travel, but this is one. It would only occur on a planet with less abundant resources, measured by how long they last during the industrial era, and only in situations where the neurology and training area of science happens to blossom late in this era. In this particular and possibly rare situation, a world-wide epidemic could have indirect effects that could collapse the civilization unrecoverably. But not only would these two requirements have to be in place, the epidemic itself would have to be at the limits of lethality, via both the disease effects and contagion. It might be that the evolution of such an infectious organism is extremely unlikely, and only by some early efforts at genetic engineering, at the level that would be possible in the later industrial era, could it arise and be, possibly accidentally, released.


Disease and Contagion in Alien Civilizations

The two aspects of epidemics are disease, what the effects of the infectious organism are within an alien's body, and contagion, which is how the infectious organism migrates from one alien to another. There are relationships between the two, but it is convenient to think of them separately at first.

When the infectious organism is inside an alien's body, that body serves as the source of sustenance for the organism. Somehow the infectious organism needs to get access to those substances that will allow it to survive and multiply. Cellular walls surround useful substances everywhere but in a few locations, such as the digestive tract and the equivalent of the blood system, meaning whatever in an alien's body transports nutrients, including oxygen, from the source locations within the body which access them from the outside. In Earth land creatures, those source locations are the lungs for oxygen and the digestive tract for everything else. So, an infectious organism that does not need oxygen directly can live in the digestive tract; otherwise it must somehow obtain its own nutrients from the body of the alien. There are nutrients in the blood system equivalent, and if the organism can somehow penetrate the walls of that system, it might find a place to survive and multiply. Thus, moving from the entry point on the alien's body to the blood stream has to be done in one way or another, and through a wound is one. Wounds should be uncommon, however, and so they would only play a part in diseases which cannot become epidemics. 

This means that the infectious organism has to have one unique capability: penetration of cell walls, either directly into cells themselves or between them into organs which have fluids, such as the equivalent of blood vessels. This can be done by toxins, which cause cells to die, or direct microchemical attack on the cell walls or their adhesion system, which binds one to another. This elementary categorization simply serves to show that the functionality of infectious organisms is not very diverse nor very complicated, and that there is no obvious reason they could not evolve on any exo-planet with animal life. It also means that there might be a multitude of types of disease-causing organisms on any exo-planet of this kind, where the next level of specification is by the type of cell in the alien's body which is attacked by the organism. 

There would be cellular defenses against infection, and also body-wide defenses, which are the equivalent of our immune system. Cellular defenses involve resistance to toxins which kill cells and resistance to penetration attacks on the cell walls and on the connections between cells. Body-side defenses involve organs within the body which produce cells specifically designed to attack and destroy infectious micro-organisms. Evolution continues to improve and adapt both sides of this battle, and while there is a degree of randomness in what evolution has produced at any given instant in time, over long ages everything gets tried that can be tried.

Every disease-causing organism would like to graduate to being an epidemic, as the numbers of the organisms would be multiplied by something quite large. Thus, evolution would also work on micro-organisms to enable their transfer from one host to another. However, there is no biological equivalent to inter-host transfer, so evolution has no way to arm the larger organisms against this in any direct way; instead defense has to be left to each large organism to defend itself against the infection. 

One piece of knowledge that is widely understood is that highly and quickly lethal organisms have a hard time spreading from host to host. There is no evolutionary advantage for a micro-organism to kill its host quickly if it can live within the host for a long time, while propagating to other hosts. If the micro-organism has evolved to overcome the first line of defense of the host, the cell walls, it can live until the immune system rises up to eliminate it. Since this takes time, measured in the rate of transfer of cells around the body of the host and the growth rate of the different types of cells that make up the immune system, there is a duration of infection that should not be shortened by evolutionary mutations within the infectious organisms; otherwise the micro-organism works to its own disadvantage. The longer the duration, the more multiplication of micro-organisms that can take place, before the immune system eventually reduces them again. 

The method of contagion plays a role here. One route for the micro-organism to spread between hosts is via death of the host and spread of the organism from the dead body of the host. If the micro-organism can live for a long time in water, any host which dies in water can spread it. If the micro-organism dehydrates the host, the host would seek water and perhaps die in contact with it. If the micro-organism infects hosts which are carrion-feeders, and cannibals to boot, this would provide another route for re-infection. This, of course, is only for wild creatures living in natural surroundings. For intelligent aliens, burial customs can influence contagion in a somewhat advanced alien civilization. Using dead animals as feed for live animals of the same species can also be involved. In such instances, lethality of the micro-organism might be higher than otherwise optimal for its propagation. 

Otherwise, the game is played by set rules, the host should live until the immune system kicks in, or would have, had the host not died from the infection. The infectious organism has to have ways to propagate, either while the host is alive and infected, or while dead and not buried, or both. These are categorized into respiration-related, touch-related including sexually transmitted, and surface-transmitted. Third parties, such as insects, can also serve as the route for contagion. For primitive alien civilizations, all of these would be in play until enough technology is gained to block them. After that, one by one they are shut down, by eliminating the insect hosts, by disinfection methods, by identification of carriers and their isolation and possibly others in special cases. So, epidemics can strike an alien civilization in analogous ways to ours, and the question about whether epidemics could be the reason alien civilizations are not visiting us depends on whether or not, at any era within the development of the alien civilization and its technology, there would be enough planet-wide transportation before anti-epidemic technology was developed. In other words, which technology stream comes first. 

Lastly, there is the question of the finality of an epidemic. Given that one happens in an alien civilization, can it recover and get back on the road to star travel, with only a delay of a generation or two or three? This might be a much more important question that the possibility of a single monstrously severe epidemic at just the right time in the technology development cycle.

Can Epidemics End an Alien Civilization?

Recently, a well-known blogger facetiously proposed a possible solution to the question of missing aliens: could epidemics have killed them off? This deserves some detailed examination.  This post and the next four all attempt to dig deeper and to provide some overview of the possibility.

Would there be infectious organisms on exo-planets harboring advanced alien civilizations? What helps us answer this is one of the main principles of alienology: convergent evolution. This principle says that the number of mutations that happens on a planet is much, much larger than the number of possible mutations; in other words, every mutation is tried out many times. Since evolution favors the more efficient at survival and reprodution, we would see on each exo-planet that has originated life and undergone billions of years of evolution, all the same niches of life filled. There might not be, at any instant in time, rose bushes on Planet X, but there would be flowers, thorns, pollination in different ways, fragrances emitted, and so on. Everything that works here would have been found and worked there, subject to lots of randomization. The principle works the other way as well, as anything that evolution could have come up with on Planet X, it could have come up with on Earth. The details are all scrambled, but the niches are occupied, the various functions are all there, and so on. 

That means that multi-cellular organisms on Planet X, where “multi” means billions, would be good homes for both infectious single-celled organisms and semi-alive RNA/DNA/protein globs which we call viruses. This has to be tempered with the realization that immune systems would have evolved in the organisms on Planet X as well, and that means that each organism there is actually a battleground between cells and viruses that would like to colonize it, and the organism's immune system cells, which are bent on getting rid of these things. The immune cells have to be able to communicate with whatever organ makes them, so they can call up large numbers when a virulent invasion hits, and so they are unable to go everywhere in the body of the organism, particularly not in the digestive system and the outside of the envelope or “skin” of the organism, plus a few other places. So infections would hit the organism in the digestive tract or on the skin of the organism. The oxygen supply system would also be an area where the immune system cannot easily patrol in large enough numbers to repel a large invasion. 

Another principle of alienology is asymptotic technology, which says that technology is an accumulation of scientific knowledge and engineering principles which builds on itself over time in a society of intelligent organisms, and has to follow some fairly well-developed paths based on how knowledge fits together and how engineering of various tools allows the next stage of technology to be developed. Iron tools allow deep mining to be accomplished; computers allow DNA to be investigated; and on it goes in a reasonably coordinated way. This way comes to an end when all technology is understood, and that does not take very many generations of aliens, perhaps something of the order of a hundred. The final stage is called asymptotic technology, meaning it is the final end or asymptote of technological progress. 

Genetics is one of the last pieces of technology to be brought under complete control of an alien civilization, as it depends on the pre-existence of much other technology to enable all the experiments that have to be done. An alien civilization which has reached asymptotic technology does not have any worries about epidemics of single-celled organisms or viruses. Any individual who become infected can be examined and equipment used to determine exactly what is the infectious agent and what does the technology library say about how to get rid of it quickly. We are not at the stage yet of knowing how to do this, but we can imagine some possibilities, none of which have to be discussed here. What is important, is that there is no mysterious illnesses possible with a sufficiently advanced alien civilization, meaning no epidemics, even locally. All bets are off on an exo-planet which has had its civilization collapse for other reasons, but one which is in the golden age of its existence will have no problems.

This means that epidemics occur only with younger alien civilizations, ones which have not yet passed the genetic grand transformation, after which genetics is wholly understood, and the technology for dealing with it developed and deployed. An alien civilization in the electronics era, the one prior to the genetics revolution, does not have the ability to analyze almost instantaneously genetic blueprints and fabricate antidotes. Instead, such a more primitive alien society must grope around, using trial and error, in the hope of finding a cure for any widespread infection or a vaccine to prevent it by giving the immune system a head start. However, if infections can produce a sufficiently widespread and catastrophic effect on such a early civilization, it would not have a chance to reach the genetic grand transformation, and would relapse into some earlier stage. 

Could an epidemic occur in an alien civilization which has not even reached the electronics or industrial age? This would be a civilization in the agricultural era, where there are few small cities, and the population is spread out over the planet in regions where agriculture is efficient and seasonality not too severe. There might be a slowly moving infection, but with very limited numbers of individuals moving from one area to another, there would not be anything to produce a catastrophe. If the infection was highly lethal, news of it would spread faster than the infection itself. If it were rarely lethal, it would simply become part of the arsenal of the resident aliens' immune system. Thus, epidemics occur in industrial civilizations that have mastered transportation to some degree, not in earlier or later ones.

So the question resolves to: can an alien civilization which falls victim, over the whole planet, to a single type of novel infection, recover from it and with some delay, return to its progress toward the further stages of technology? If the infection is sufficiently lethal, its spread is inhibited. If the infection is not very lethal, it becomes part of the immune system's library of known invasive organisms. Exactly what lethality is needed for a collapse after which there is no recovery, even after a century? If it is too high when it arrives, carriers do not carry it far before expiring. However, if there is no immune system response possible, in other words, if the attacking organism can defeat the immune system of the individual aliens so they do not develop immunity to it, and can then invade and re-invade and re-invade until lethality results, but with plenty of transmission between individuals during the intermission between successive invasions, this might do it. So, an epidemic which attacks the immune system or which is 'immune' to the immune system, which damages individuals on the first attack instead of killing them leaving them more vulnerable to future infections, and which is easily contagious, might eliminate the alien civilization, and prevent it from ever building starships and coming to Earth. Such an infective organisms, a triple-headed threat, might be stopped with social measures in an alien civilization in the industrial era, but that is another question to be answered later.