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ET v. Earth Pathogens
Will ETs Kill Us or Vice Versa?

In H. G. Wells’ 1898 novel The War of the Worlds the seemingly invincible invading Martians are wiped out by Earth’s microbes, against which they have no immunity. In Michael Crichton’s 1969 novel The Andromeda Strain the opposite happens: the people of a small town in Arizona are annihilated by a microorganism accidentally brought to Earth in a crashed satellite. Both novels share the thesis in common that microbes from any given planet will be brutally inimical to the denizens of another planet, who have no immunity to these alien pathogens. In Gordon R. Dickson’s 1987 novel Way of the Pilgrim, the Aalaag alien conquerors of our planet are immune to Earth’s microbes because their biochemistry is so different from that of the native pathogens. Who is right, H. G. Wells, Michael Crichton, or Gordon R. Dickson?

Andromeda Strain poster

To some degree, each of these authors probably had a piece of the truth in their fictional works. Assuming ETs coming to Earth are carbon-based life forms that evolved in a watery medium, it seems likely that natural prebiotic chemical reactions would produce the same basic biochemical building blocks on their home planet that gave rise to life here on Earth. This assumption is based on well known research such as the 1952 Stanley Miller and Harold Urey experiment, which produced as many as 20 different amino acids from electrical discharges into a reducing atmosphere of methane, ammonia and water vapor; the 1961 experiment by Joan Oro, which produced adenine, among other compounds, from hydrogen cyanide, and amino acids from hydrogen cyanide and ammonia;1 and the 2008 volcanic discharge experiment.2 Beginning with the same building blocks, such as amino acids and cyclic compounds such as adenine, carbon-based life developing in water on other planets, would, given the natural chemical processes observed in these experiments, likely have generated a basic biochemistry similar to ours. That is, bodies made up of, among other things, proteins and nucleic acids. Hence, they would likely be potential food sources for Earth’s microbial pathogens.

War of the Worlds cover

However, there are certain factors we need to consider before making the sweeping generalization that either visiting or invading extraterrestrials would be sitting duck hosts for our microbes. Three of these are: possible differences in biochemistry between terrestrial and extraterrestrial life forms with respect to the “handedness” of their molecules, the possibly arbitrary nature of our genetic code, and the transmission of certain diseases by insect and arachnid vectors.

Right or Left Handed ETs?

Just as our right and left hands are mirror images of one another, and just as we can’t fit our right hands into gloves made for the left hand, so also are certain organic molecules mirror images of each other. Among these are amino acids, the building blocks of proteins. Proteins are polymers—giant molecules made up of repeating subunits (much smaller molecules called monomers). Polymers can be compared to a brick wall, a large structure made up of the repeating subunits of many individual bricks. There are 20 different types of amino acids (monomers) that are chemically bound together to form our proteins, which are the principle actors in the chemistry of life. Proteins make up our enzymes (organic catalysts), as well as our bones, cartilage, muscles, skin, internal membranes and organs, and, along with complex lipids, our nerve tissue; in short, just about everything of which we are made.

Andromeda Strain poster

The carbon atoms of which we are made are bonded to four different atoms or molecular groups and are called asymmetrical carbon atoms. Molecules containing these asymmetrical carbon atoms—such as amino acids—can be mirror image isomers. These mirror image isomers, when crystallized, rotate polarized light shined through them either to the left or to the right. For this reason they are called optical isomers and are labeled “L” for levo (left)-rotary or “D” for dextro (right)-rotary. When amino acids are created in laboratories, roughly even numbers of L and D-amino acids are produced. However, all proteins here on Earth are made up of L-amino acids. There seems to be no reason they could not as easily have been made up of Damino acids, so on another planet life could have developed with a biochemistry that is the mirror image of our own. Should this be the case, it might be that the proteins of extraterrestrial life forms would not be digestible by our microbes (nor ours by theirs) and their amino acids might not fit onto the surfaces of the enzymes of terrestrial life forms to either be broken down or attached to RNA molecules to be formed into proteins.

The Arbitrary Nature of Our Genetic Code

While there are over 20 different amino acids that make up our proteins, there are only four different cyclic compounds that are found in nucleic acids. These are adenine, guanine, thymine and cytosine in DNA. In RNA thymine is replaced by uracil. RNA also has a different five-carbon sugar, ribose, from DNA. The five-carbon sugar in DNA, deoxyribose, differs from ribose in having one less oxygen atom. DNA is copied onto RNA, which then codes for the sequence of amino acids in our proteins. Since there are only four cyclic compounds and 20 amino acids, it takes a sequence of three of these cyclic compounds, called a codon, to code for one amino acid. For example, a sequence of three uracil nucleotides in an RNA chain codes for the amino acid phenylalanine. Since there are 64 possible combinations of sequences of any three of the four cyclic compounds, the genetic code is redundant, with many amino acids coded for by as many as three triplet codons.

War of the Worlds cover

Curiously, the molecules of RNA that pick up the amino acids to be placed in sequence have the codon at the opposite end of the molecule from the end that picks up the amino acid. For example the U-U-U codon for phenylalanine does not interact with phenylalanine in any way. Rather, the U-U-U codon and the attached phenylalanine molecule each seem to fit onto the surface of an enzyme. It would seem that the genetic code, common to all forms of life on our planet, is entirely arbitrary, and that an entirely different code could prevail on another planet. In fact, seemingly arbitrary substitutions of different codons occur even on Earth. For example, in human mitochondrial genes (mitochondria, the cell’s “powerhouses,” have their own genes, separate from the genes of the cell’s nucleus) the codon AUA (adenine, uracil, adenine) codes for the amino acid methionine, rather than for isoleucine, as it does in the RNA code derived from the cell’s nucleus. Also, in the mitochondrial code, UGA (uracil, guanine, adenine) codes for the amino acid tryptophan, while in our cells’ nuclear RNA, UGA is a stop codon, signaling a break in a chain of amino acids.3 This substitution is also true of the genetic code of the bacterium Mycoplasma, while in ciliated protozoa both UAG and UAA, two other stop codons in most life forms, code instead for the amino acid glutamine.4

If the genetic code of extraterrestrial genes were completely different from ours, viruses, which reproduce by hijacking our protein-making machinery to read the viral genetic code to make new viruses, would not be able to infect extraterrestrials bearing a different genetic code. The triplet code of the viral nucleic acid (either RNA or DNA) would be read by the host’s protein-making machinery as coding for different amino acid sequences from those of the viral proteins. Likewise, we would be immune to any viruses carried by extraterrestrials visiting planet Earth.

Opportunistic Bacteria and Fungi

Given that our viruses might not attack aliens with differently coded DNA and RNA, and that many of the diseases that attack human beings are transmitted by insects, ticks and lice that suck our blood, and might not find E.T.’s blood appetizing or even edible, do visiting or invading extraterrestrials get a free pass? Hardly. There are, along with the more specialized parasites that infest planet Earth, a number of microbes, particularly bacteria, that infect a wide variety of organisms, both plant and animal, and that are extremely opportunistic. Three genera of opportunistic bacteria that might plague E.T. come readily to mind: Staphylococcus, Streptococcus and Pseudomonas. All of these have in common a number of different species that attack a number of different types of organisms, increasing resistance to antibiotics and the ability to subsist on a wide variety of compounds in a number of different environments.

War of the Worlds cover

Consider Staphylococcus. Necrotizing fasciitis, commonly referred to as flesh-eating bacteria syndrome, is often caused by methicillin-resistant Staphylococcus aureus (MRSA). While this disease is comparatively rare, it highlights the marked resistance of S. aureus and other Staphylococcus species to antibiotics, as well as the opportunistic nature of this form of bacteria. Various species of Staphylococcus infect birds and mammals. It would seem likely that extraterrestrial endothermic (warm blooded) species could also act as their hosts.

Streptococcus infects humans and certain other mammals. Like Staphylococcus, it is ubiquitous, causes a variety of different types of infections and can be spread by airborne spores.

While the fact that both Staphylococcus and Streptococcus pathogens are limited to animals would seem to indicate they might not infect an extraterrestrial life form whose chemistry is far different from that of any animal, various species of Pseudomonas are pathogens on both plants and animals, and many can as well metabolize aromatic hydrocarbons, such as toluene and ketones. Various strains of Staphylococcus and Streptococcus can also metabolize hydrocarbons.5 Bacteria that can eat hydrocarbons, as well as organic compounds from living things, should be able to do the same to extraterrestrial flesh. Also, like S. aureus, various strains of Pseudomonas have developed increased resistance to antibiotics. Various common forms of fungi, particularly Candida and Aspirgillus, are also capable of metabolizing hydrocarbons, and are used in bioremediation of petroleum wastes.6 Again, if they can eat hydrocarbons, they may well be able to eat extraterrestrial flesh.

What E.T. Might Bring Us: The Native American Experience

In the 1730s French explorers, reaching the upper Missouri river, came in contact with the Mandan, a Siouan-speaking Native American tribe. At that time they estimated the Mandan population at about 15,000. By the time the Louis and Clark expedition reached Mandan territories in 1803, they found many abandoned villages, apparently wiped out by smallpox. Nevertheless, the Mandans’ estimated strength was about 3,500, until two smallpox epidemics devastated them in 1837 and 1838. By 1949, when the Smithsonian Institution published a volume titled North American Indians, there were only 263 Mandan left, sharing a reservation with another tribe, the Hidatsa.7 It is significant that the Mandan never entered into armed conflict with either the United States army or the white settlers.

War of the Worlds cover

While some of their losses were due to attacks from other tribes, these came mostly after the Mandan population was already devastated by smallpox epidemics. Thus, after repeated smallpox epidemics the Mandan population was reduced to less than two percent of it’s original estimated strength. While the numerical strength of native peoples in the western hemisphere varied with respect to the effects of smallpox and other diseases brought by European settlers, the overall impact of such diseases was devastating. An outbreak of smallpox decimated the Aztecs in 1520 when they were about to overwhelm the Spaniards under Cortez by sheer numerical superiority. The Inca Empire was likewise disrupted by smallpox in advance of Pizarro’s expedition. Estimates of the percentage of the pre-Columbian population of the Americas lost to diseases inadvertently introduced by European contact go as high as 95 percent.8 To some degree, finding a continent so largely depopulated in advance of settlement led to the conviction, voiced by John L. O’Sullivan in 1845, that it was America’s, “manifest destiny to overspread the continent allotted by Providence,” though it wasn’t God, but rather smallpox, influenza, measles and even chickenpox that emptied the continent to colonization.

Assuming either the vanguard of an alien invasion or the first friendly extraterrestrial contact involves a relatively small number of extraterrestrials who might be infected by our microbes and might be quarantined before they could spread the infection to their home planet, might we not be more at risk from germs they introduce to Earth than they are from ours? This is the thesis of Michael Crichton’s novel The Andromeda Strain. Of course, aliens bent on conquest might well seed the Earth with pestilent spores before invading. However, a civilization capable of interstellar travel would probably have solved its material and energy problems in the process of developing, first an interplanetary, then an interstellar civilization. The energy required to put a fleet across interstellar space would probably only be available to a civilization that had harnessed controlled fusion as an energy source. Arriving in our solar system, such a fleet could harvest carbon and hydrogen from the moons of the outer planets, oxygen, aluminum and silicon from aluminosiliicate rocks in our asteroid belt, and metals such as iron, nickel, copper and cobalt, from metallic asteroids in the asteroid belt. They could even use these raw materials to create space habitats in which to live comfortably in our solar system, all without having to invade Earth. As such, an interstellar culture might well have neither the need nor the desire to conquer us. That being the case, such a culture might also rigorously guard against interstellar cross-contamination by microbial pathogens.

Skeptic 22.1 (cover)

This article appeared in Skeptic magazine 22.1 (2017).
Buy this issue

Another factor to consider regarding the invasion of alien pathogens is that our culture is far more technologically sophisticated than were those of the pre-Columbian Americas. Consider the impact of the AIDS pandemic worldwide. Globally, 36.7 million people are now infected with the human immunodeficiency virus (HIV).9 The global population is now nearly 7.5 billion,10 meaning that, for all its virulence, the HIV virus now infects less than one percent of the global population. A far greater global scourge is malaria. According to the World Health Organization (WHO), over 210 million people are now infected with malaria worldwide,11 which amounts to less than three percent of the world’s population. It is notable that, in the case of both AIDS and malaria, sub-Saharan Africa is the area most impacted. It is also the area with the least sophisticated infrastructure of any large region of the world. This supports the hypothesis that our higher level of technology and medical infrastructure would significantly blunt the onslaught of any extraterrestrial plague inadvertently introduced by alien visitors. A constellation of many and varied microbes set loose upon our planet by uncaring conquerors might well overwhelm our civilization’s infrastructure; but, again, thinking pragmatically, conquest of inhabited planets in other star systems might not be of interest to any interstellar civilization. END

About the Author

Tim Callahan is religion editor of Skeptic magazine. His books include Secret Origins of the Bible, and Bible Prophecy: Failure or Fulfillment? both published by Millennium Press. He has also researched the environmental movement, and his article “Environmentalists Cause Malaria! (and other myths of the ‘Wise Use’ movement)” appeared in The Humanist. He has co-authored UFOs, Contrails and Aliens: What Does Science Say? currently in press.

  1. Oro, J. and S.S. Kamat. 1961. “Amino-acid Synthesis from Hydrogen Cyanide under Possible Primitive Earth Conditions.” Nature. 190, 4774, 442–443.
  2. Johnson, Adam P., H. James Cleaves, Jason P. Dworkin, Daniel P. Glavin, Antonio Lazcano, Jeffrey L. Bada. 2008. “The Miller Volcanic Spark Discharge Experiment.” Science. 322, 17, October.
  3. Barrel, B. G., A. J. Bunger and J. Drouin. 1979. “A different genetic code in human mitochondria.” Nature. 282, 8 November.
  4. Jukes T. H., Osawa S., Muto A., Lehron N. 1987. “Evolution of anticodons: variations in the genetic code.” Cold Spring Harbor Symposium on Quantitative Biology. 769–76.
  5. Yen, K. M., et al. 1991. “Cloning and characterization of a Pseudomonas mendocina KR1 gene cluster encoding toluene-4-monooxygenase.” Journal of Bacteriology. Sept; Onaca, Christina, Martin Kieninger, Karl H. Engesser, and Josef Altenbuchner. 2007. “Degradation of Alkyl Methyl Ketones by Pseudomonas veronii.” Journal of Bacteriology, May; Jyothi K., et al. 2012. “Identification of Hydrocarbon Degrading Bacteria by Molecular Characterization.” Helix. 2: 105, 111, March 1; Patel, Priyank, et al. 2015. “Biodegradation of Petroleum Hydrocarbons by Newly Isolated Bacterial Strains Acinobacter spp. & Streptococcus spp. SK.” Conference: 8th National Level Science Symposium, Christ College Rajart, Gujarat, India February 22.
  6. Hung-Soo Joo, Pius Noegwa, Makoto Shoda and Chae-Gun Phae. 2008. “Bioremediation of oil contaminates using Candida catanulata and food waste.” Environmental Pollution. 156/3, December; Maruthi, Y Avasn, et al. 2013. “Apergillus flavus: A potential Bioremediator for oil contaminated soils.” European Journal of Sustainable Development.
  7. Palmer, Rose A. 1949. North American Indians: The Smithsonian Series, Vol. 4 Washington, D.C.: The Smithsonian Institution, 299.
  8. Diamond, Jared. 1997. Guns, Germs and Steel. New York: W. W. Norton, 77, 78.


  1. J. Gravelle says:

    Technically, we’re ALL ETs…


  2. Ramey Zamora says:

    Excellent arguments and expositions, Tim. There’s certainly plenty to consider and ponder when it comes to off-Earth based life forms. Agree that very advanced societies capable of interstellar/interplanetary travel would have no interest in Earth except perhaps for curiosity’s sake. In fact, when we try to imagine their means of energy, we can only come up with the ones we already know about. My guess? Their method of travel will harness forces completely unimagined by our early 21st century selves.

    But, it must be assumed from the get go that our viruses and bacteria, introduced onto moons and planets in our solar system, could result in havoc before the native life forms were able to adapt. We already have protocols in place attempting to mitigate if not eliminate the possibility of contaminating other worlds. But — it only takes one, doesn’t it?

    Thanks for the great article.

  3. Tzindarro says:

    The isolation of North America before 1492 is greatly exagerated. There were plenty of contacts before the modern era of European invasions began. Not mass migrations perhaps, but more than enough to introduce Eurasian diseases if the population had been so vulnerable to them as is usually believed. The Mandan, in fact, were of mixed ancestry already, having interbred with Welsh colonists who came over in 1170 A.D.

    The most likely cause of the decimation of Amerindian populations in contact with Europeans is alcohol, not contagion.

    • Ken Farnsworth says:

      “The Mandan, in fact, were of mixed ancestry already, having interbred with Welsh colonists who came over in 1170 A.D.”

      This is hardly fact. While it is a persistent myth, there doesn’t appear to be any evidence that it actually occurred. There is no linguistic or DNA evidence whatsoever. Not sure why you would favor this unsubstantiated story over the smallpox epidemics stories, which are well attested. It’s always interesting to me why someone would accept some stories on the flimsiest of evidence, yet reject other stories that are much more substantiated. Fascinating.

      • Tzindaro says:

        The isolation of the Americas is a myth. There was always plenty of contact across both the Atlantic and the Pacific throughout the time the Americas have been inhabited. There are Roman artifacts found in Virginia, Irish beehive domes in New England, more than 400 Old Norse words in the Algonquian language, grafiti carved on a rock near Boston signed and dated by Henry Sinclair, Earl of Orkney, in 1395, Mongol spurs, bits, and stirrups found in California, and deep copper mine pits in Tennessee, where the Cherokee say they fought with some strangers passing by because these strangers dug in the earth, which was against the religion of all Amerindian tribes. The claim that the Indians were especially vulnerable to Eurasian diseases because they had been isolated so long they had had no exposure to them simply does not hold up.

        On the other hand, they WERE and still are, especially vulnerable to alcohol, which was in frontier times and still is, along with associated fighting, the number one cause of death among all Amerindians. But that has nothing to do with lack of immunity to contagion by microbes.

        And, by the way, the Souixian people were an Eastern Woodlands tribe at the time they first contected whites. They were only forced out onto the plains by their enemies, the Chippawa, when the later got guns from white fur traders AFTER the smallpox epidemic had depopulated the plains area of Mandans and others so they could move in.

        And the name, Mandan, is the name of a Celtic god, the one the Isle Of Man is named after. And there are written records in Wales of a Prince Modoc who sailed out to sea with 300 followers in coracles, the traditional Celtic leather boats, and was never heard from again. The Mandan used a type of boat no other tribe in North America used, made by streching bison hide over a wood frame, exactly the way a coracle is made, as still used today on the west coast of Ireland.

  4. Bob Pease says:

    If it happens we cannot/will not leave the planet

    Hopefully technology will find a way to deal with the Orson Wells scenario.

    This ranks low in the list of doomsday scenarios

    Global warfare among Dominionist Religions who
    believe that they must feed their people first is very high on the list of doomsday scenarios

    Dr, Sidethink Hp. D.

  5. Bob Jase says:

    The Martians are the exception in the menu of ets. There is a chance that we would have origins, possibly even shared subsequent exposure to trans-planet microbes.

  6. Barbara Harwood says:

    It is entirely possible that any IT’s are so different from us that they might prefer one of our unoccupied planets because it is closer to what they are used to. They might simply tune into us for entertainment. Let’s hope that they have a sense of humor.

  7. BillG says:

    My naive speculation on any ET visit would not be biological but perhaps some type of von Neuman self-replicators – at least initially.
    Of course we could still have an “Andromeda” like strain on us lowly humans.

    Currently, I think I will lose sleep on other dire issues.

  8. Richard Board says:

    The statement accompanying the photo claims that the cysts and eggs “survived” the breakup and crash of Columbia. How was that confirmed? Couldn’t these creatures have been found on the shuttle’s remains after landing on Earth? What certainty is there that they survived launch, orbit time and reentry?

  9. awc says:

    I wonder why the genome is the only means to transfer and store information?

    I would have thought a few means to pass information down through through generations would happen.

  10. OldNassau says:

    1. “than two percent of it’s original”: I wonder if ET’s will know the difference between “it’s” and “its”.
    2. Another anthropocentric argument assuming we can sense them, and they, us. I remember reading a sci-fi short story in which the human landed on a planet inhabited by jumping pogo sticks. There was no interaction.
    3. For all we know, ghosts are ET’s, sensed by each other, but ethereal to us. And vice versa. Perhaps dark matter is mass to them, and our mass, their dark matter.
    4. In which case, not to worry.

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