Note: For the purpose of this question I define a pathogen to include viruses and/or bacteria. Edit: I don't exclude other biological agents such as fungi that might have the same effect.


A pathogen or infectious agent is a biological agent that causes disease or illness to its host.


Sometimes we hear of a potential threat to humanity from a 'killer' virus, for example bird-flu. https://www.webmd.com/cold-and-flu/flu-guide/what-know-about-bird-flu#1

There have been plagues that have killed vast numbers of people but none have resulted in actual extinction. https://www.mphonline.org/worst-pandemics-in-history/

HIV Aids, Bird-Flu and others have affected more than one species.


In theory could a microbe arise that could completely wipe out an entire species (in particular humans) or more importantly all mammals (and maybe other species as well).

Clearly mammals have not all been wiped out so far because we are still around. My question is, could it conceivably happen given what we currently know? If not, what would prevent it?

Edit I should repeat that I want it to be an agent that directly infects mammals by getting into their bloodstream or other bodily systems - not one that affects them indirectly.

  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$
    – L.Dutch
    Commented Jan 18, 2019 at 3:35

10 Answers 10


Yes, but not in the way you are thinking.

One way this could happen is if some killer fungus starts growing unchecked throughout the planet, on land and sea. By killing all plant life it would destroy every other ecosystem, so it would wipe out all mammals. It would also practically wipe out all other members of the animal kingdom. This is part of the plot of Sid Meyer's Alpha Centauri.

Another way is if some methane-producing microbe starts spreading unchecked. This may have already happened:

The Permian–Triassic (P–Tr or P–T) extinction event, colloquially known as the Great Dying, the End-Permian Extinction or the Great Permian Extinction, occurred about 252 Ma (million years) ago, (...) It is the Earth's most severe known extinction event, with up to 96% of all marine species and 70% of terrestrial vertebrate species becoming extinct. It is the only known mass extinction of insects. Some 57% of all biological families and 83% of all genera became extinct.


Suggested mechanisms for the latter include (...) a runaway greenhouse effect triggered by sudden release of methane from the sea floor due to methane clathrate dissociation according to the clathrate gun hypothesis or methane-producing microbes known as methanogens.

And if methane won't do, bacteria may deplete the oceans of oxygen. Then:

A severe anoxic event at the end of the Permian would have allowed sulfate-reducing bacteria to thrive, causing the production of large amounts of hydrogen sulfide in the anoxic ocean. Upwelling of this water may have released massive hydrogen sulfide emissions into the atmosphere and would poison terrestrial plants and animals and severely weaken the ozone layer, exposing much of the life that remained to fatal levels of UV radiation. Indeed, biomarker evidence for anaerobic photosynthesis by Chlorobiaceae (green sulfur bacteria) from the Late-Permian into the Early Triassic indicates that hydrogen sulfide did upwell into shallow waters because these bacteria are restricted to the photic zone and use sulfide as an electron donor.

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    $\begingroup$ Another example is the development of photo synthesis in early life on earth. This caused the release of massive quantities of oxygen into the atmosphere, killing off almost every older creature as those were incapable of handling the now extremely toxic and corrosive environment that those proto plants had created for themselves. $\endgroup$
    – jwenting
    Commented Jan 16, 2019 at 12:16
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    $\begingroup$ Although on the face of it this is a good answer, it doesn't truly reflect the question. I'm looking for an infection that attacks mainly mammals. There are indeed fungal infections that infect mammals so that would count. However a fungus that only affects other biomes but only affects humans/mammals indirectly doesn't really fit. $\endgroup$ Commented Jan 16, 2019 at 19:57

If by single pathogen you mean a strain of virus or bacteria and not a single specific bacterium or virus, then Yes. We already have an existing example - Rabies. Rabies only affects mammals. If your strain is easier to transmit via multple methods (air, water, soil) and harder to kill (e.g. boiling water will not kill it) with widespread travel throughout your world, you should be able to spread the virus worldwide. Eventually, even the most prepared survivalists will run out of stored food and potable water.

In unvaccinated humans, rabies is almost always fatal after neurological symptoms have developed.

Vaccination after exposure, PEP, is highly successful in preventing the disease if administered promptly, in general within 6 days of infection. Begun with little or no delay, PEP is 100% effective against rabies. In the case of significant delay in administering PEP, the treatment still has a chance of success.

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    $\begingroup$ 'this answer should be backed up' - Name a mammal immune to rabies. Anyone? $\endgroup$
    – Mazura
    Commented Jan 17, 2019 at 0:19
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    $\begingroup$ Oh my. So now it's bat planet? -{shivers}- $\endgroup$
    – Mazura
    Commented Jan 17, 2019 at 2:11
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    $\begingroup$ @chasly from UK, it's your world you are building, make it mutate. $\endgroup$
    – B540Glenn
    Commented Jan 17, 2019 at 14:09
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    $\begingroup$ @aherocalledFrog Opossums actually very rarely carry rabies! While they can carry it, like all mammals, their body temperature is naturally a bit low for the rabies virus, so they're unlikely to develop it. $\endgroup$
    – Cooper
    Commented Jan 17, 2019 at 19:33
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    $\begingroup$ @L.S.Cooper: I've got books claiming that bats carry it but are themselves resistant to it, which would result in at least some of them coming through the plague. $\endgroup$
    – Joshua
    Commented Jan 17, 2019 at 20:04

Patch Actinomycetes (a kind of soil bacteria) to carry cDNA for Ricin.

Uncontrolled genes in bacteria tend to be on for some reason. The entire field of genetic research depends on this. Even if this were not the case, rigging up an activator is easy.

The bacteria now manufactures Ricin, which is a broad-spectrum deadly poison that functions by deactivating eukaryotic ribosomes. This now makes them a true pathogen with the unusual reproductive mode that it gains energy by killing its hosts and then decomposing them.

Actinomycetes already spread through the air. This patch would make the spores contacting dirty skin a plausible transport route. A patch like this probably results in the bacteria using Ricin as an endotoxin, which is still deadly enough once enough affected dust is inhaled or ingested. It won't take particularly dirty food to get a fatal dose.

Correctly unleashed, this would be an excessively deadly device, and obviously far too dangerous to be any kind of weapon other than a doomsday device. Soil dust crosses oceans so a quarantine would not be particularly effective. Once this thing gets rolling, it's going to require ridiculously overpowered intervention to stop it. I want to say supernatural but if I asked how to stop this on worldbuilding somebody will come up with something.

  • $\begingroup$ I didn't fully get what you meant by 'one hemisphere' being doomed. Could you elaborate as to why one hemisphere could be safe? $\endgroup$ Commented Jan 16, 2019 at 23:02
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    $\begingroup$ @chaslyfromUK: I don't see any way it can cross an ocean by itself once humans institute quarantine. $\endgroup$
    – Joshua
    Commented Jan 16, 2019 at 23:03
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    $\begingroup$ crossing oceans on soil dust? $\endgroup$
    – bukwyrm
    Commented Jan 17, 2019 at 16:01
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    $\begingroup$ Soil dust is a good answer for it being carried from one continent to another. That really happens in our world [link] (nesdis.noaa.gov/content/saharan-dust-blows-across-atlantic) $\endgroup$ Commented Jan 17, 2019 at 16:13
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    $\begingroup$ @GaboSampaio: Updated. Now it's worse. $\endgroup$
    – Joshua
    Commented Jan 17, 2019 at 16:20


Mammals (Mammalia) are structured (currently) like this:

  • Subclass Yinotheria
    • Infraclass Australosphenida
      • Order Monotremata
  • Subclass Theriiformes
    • Infraclass Holotheria
      • Superlegion Trechnotheria
      • Legion Cladotheria
        • Supercohort Theria
          • Cohort Marsupialia
          • Cohort Placentalia

That is a large target.

Genetically speaking the Platypus (a monotreme under the Order Montremata) shares roughly 82% of its genes with other mammals, and has roughly two thirds the number of genes as found in a Human.

In comparison a Chimpanzee differs to a Human genomically by about 3%, although there are 35 million single nucleotide edits across the full DNA strands. There are viruses, bacteria and parasites capable of cross-species infection between humans and apes. So there is some plausibility that one of these could be evolved/engineered to target the Cohort Placentalia.

For most people wiping out placentals mammals is enough to satisfy your requirement.

However Marsupials are also mammals. They are endemic to Australia, South and North America, and a few islands. Monotremes are known to exist on the Australian continent and Tasmania alone. At least in these places a disease that wipes out mammals would need to include these creatures.

Avian Influenza is an example of a virus capable of jumping animal classes, specifically Avian to Human and back. This does give credence to a virus being capable of infecting any mammal, and being carried by another species. Birds are good for this as they can carry the disease across geographical dividers. Unfortunately they would not be ideal agents for infecting most sea-mammals.


This pandemic mammalian disease will also have to contend with a wide-scale of environments. Mammals are widely dispersed and occupy numerous environments: ocean, stream, land, cave, mountain, valley, savannah, jungle, and sky. They live in the Arctic circle all the way through to the Antarctic circle.

While this might not seem important, when not within a host, the disease must be capable of maintaining itself in the larger environment, and either seeking or waiting for another suitable host, otherwise it essentially dies with its host.

Disease Viability

A disease that co-evolves with its hosts cannot kill all of its hosts. That would be a bad survival trait causing the disease and the hosts to die out - extinction.

A disease that jumps from one type of Host to another, has some latitude initially. After all it has a conserved host base, and the new host has zero defenses. This might eliminate a specific target host if all of those hosts were to become infected simultaneously. However nature has large barriers that tend to prevent simultaneous infection of all novel hosts - like mountains, rivers, and oceans. Thus through chance eventually the new host type will either die out, or some of their number will survive, leading back to the disease not killing all hosts.

Sexual Reproduction and genetic variability

Because Hosts have been evolving for millions of years against a back ground of infection and parasitisation. They do have a few tricks. One is sexual reproduction. This recombines genes from two parents reshuffling the genome and essentially the entire structure of the DNA. This decreases the likelihood that a disease that is effective against either parent, will be similarly effective against the off-spring.

This tends to force diseases to focus on conserved genetics, and the conserved proteins and complexes that are subsequently produced. Fortunately this allows the immune system of the animal to learn and focus defenses around these conserved areas.


Of course diseases can rage uncontrolled. There is a problem though. They need fresh meat (literally) to maintain the plague conditions.

Humans recently figured out how to achieve this with the invention of cities, and co-habitation with domesticated/pest animals. You can see this through the history books from at least Egypt on through the European medieval cities. These places were essentially death traps for humans and animals alike with disease jumping between species, but not running out of fresh hosts because more would be brought in (be they human, or other animals). The black plague is a good case in point. It was spread by the inter-city trade, but once entrenched in a city, it could last for years.

Unfortunately this mode of plague sustenance will not work for killing all mammals. Largely because many will not migrate in a sustained manner to a city, or even its cave/oceanic equivalents. But also because the Humans have learnt how to institute quarantines, and other health interventions which suppress plagues until they naturally die off.


It is theoretically possible to evolve/engineer a bacterium/parasite/virus that could be used to wipe out a given population of a particular species, or even several species/orders/classes. Numerous examples exist currently.


  • There is a chance that sexual reproduction will have created at least one individual that can survive being infected
  • The animals are not sufficiently concentrated to ensure that all hosts are simultaneously infected, or continual reinfected should the initial infection be defeated.
  • The specific disease will have to focus on the most highly conserved areas of the genome, where the animals will have highly advanced immunological defenses.
  • Humans will naturally institute quarantines, and other health measures to reduce infection and severity.

Can it happen?

In short:

  • Wiping out a group of individuals is possible - it happens often enough see the spanish flu.
  • Wiping out a species will be difficult, but not unachievable. A new species in a small environment would be simplest, any geographically diverse or separated species complicates maters.
  • Wiping out a Cohort, Order, or Class is supremely difficult as conserved sequences are already well tested for vulnerabilities, but if such did exist universal extinction would require a near simultaneous infection, and a plague supporting environment. Essentially a high concentration of all targeted hosts, which maximises reinfection of any survivors in order to overwhelm immunological defenses, and supports chances for cross-species infection from a carrier host type.
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    $\begingroup$ One thing that is IMHO worth stating explicitly is that mutations in the pathogen that will decrease lethality will be evolutionary favored if there is enough time (not killing your host means the host spreads the disease for longer and there is less selection pressure on the host to gain immunity). Since bacteria and viruses evolve quickly, it is quite likely such mutations will occur and outcompete the initial pathogen (unless all hosts were infected simultaneously). $\endgroup$ Commented Jan 17, 2019 at 12:35
  • $\begingroup$ @MartinModrak: I found away around it to make the pathogen prefer to be as lethal as possible. $\endgroup$
    – Joshua
    Commented Jan 17, 2019 at 15:31
  • $\begingroup$ Add to environments, the disease has to get to those environments, small islands, hibernating polar bears, plateau living wolves, it has to reach them all in a timely fashion. $\endgroup$
    – John
    Commented Jan 18, 2019 at 5:50

No, it is not conceivable that this would happen although it is perhaps possible in the strictest sense.

Every known pathogen has limited host range, and no known pathogen kills every individual of even a single species[1], so a pathogen that is able to effectively attack every mammal is some orders of magntiude more broad spectrum and pathogenic that anything we have ever encountered.

This should not surprise us, the fact is that multicellular organisms all have complex immune systems designed to protect them, and mammals - in particular - have some of the best and most complex. These mechanisms are extremely effective at wiping out incoming pathogens and mind-bogglingly flexible in what they can attack. Not only that, but the mechanisms by which pathogens attack their hosts are incredibly specific. It is common for viruses and bacteria to rely on binding to specific proteins in order to enter cells, and these binding interactions are so specific that even a single amino acid change can be enough to prevent their function.

You mention HIV and bird flu, but even these pathogens are only able to infect only a couple of species, and - in the case of HIV - it took blood-borne infection to initially cross the species barrier followed by in-host evolution. More common trans-species infections are things like Campylobacter jejuni or Salmonella which cause food poisoning in humans. But these organisms while they cause gastroenteritis are unable to actually invade the host's cells or organs in almost all cases.

You asked for a specific reason, it is this: Evolution. Any branch of multi-cellular life that could be rendered extinct by a pathogen as you suggest got weeded out long, long ago. Those that are left are those that are the descendant of those that were effective in the never-ending fight against pathogens.

[1] Baring species that have very few, highly inbred individuals, e.g. the Tasmanian Devil.

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    $\begingroup$ This is the best answer, in my opinion, because it brings up the notion that surviving and evolution is a continuous fight. The chance that a pathogen evolves so it could defeat EVERY mammal immune system in existence (let alone contagion means and environmental resistance) is so improbably small, considering the laws of biochemistry and the entropic principles of evolution, that for all practical effects it is impossible. $\endgroup$
    – lvella
    Commented Jan 17, 2019 at 13:30

The variety and genetic diversity of species and of each individuals tends to prevent that. Zoonoses are diseases that can infect animals and human they sometimes share the same mechanism of infection between different hosts like HIV for example. But in others case they develop hosts specific solution for infection. in both case variation may protect you from infection, they are for example genetic condition that are known to protect you from HIV infection (see the crispr baby for example). Such a Virus that would take out all mammmals must either target for infection and killing a very conserve characteristic that all mammalls posses without exeption or be adapted to all mammals, in either case this seems highly unlikely.

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    – Gryphon
    Commented Jan 16, 2019 at 14:44

Yes, viruses and bacteria can infect multiple species, even not closely related. If a hypothetical pathogen, lets call it pigeon pox, mutates, and gains ability to infect some tissue or organ shared by all mammals e.g. placenta, and as a side effect it causes the death of a foetus, it can potentially wipe out all mammals in a generation. It will not be easily contained like ebola, because its main reservoir is still a pigeon. If it survives in female reproductive tracts after killing the foetus not only it is preventing any future pregnancy but may also spread among mammals directly as STI. Pigeons will still be happily flying, coughing an sh**ting in our towns when we're gone because for them the virus only causes mild rash around cloaka.

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    $\begingroup$ Not all mammals have a placenta, c.f. marsupials and monotremes. $\endgroup$ Commented Jan 16, 2019 at 15:06
  • $\begingroup$ yes, it's an example. pigeon pox is not real either as far as i know. the original question asks about wiping out a species or more. most of mammals have placenta. if we want to make sure the pathogen kills them all, choose some other tissue, or maybe even single protein that all mammals share and the bacteria likes eating. also humans will probably find out some way to cure or prevent the disease before going extinct but that's up to the story that's being told. $\endgroup$
    – Milo Bem
    Commented Jan 16, 2019 at 15:21
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    $\begingroup$ I like this basic idea, but even more effective would be a pathogen that turns the fetus infertile. This would be a disease that has a latent period of years, giving a very long gestation period ensuring that even a program of killing all pigeons will be too late to fight the genocidal disease. $\endgroup$ Commented Jan 16, 2019 at 15:38
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    $\begingroup$ as a story, yes, much more interesting. but scientifically it's less likely to succeed because infertility is rarely complete. if 99.99% of humans are 99.99% infertile, and the remaining 0.01% are 90% infertile that means a near extinction level but it leaves a next generation aware of the problem and at least partially resistant to it. So depending on the story we want to tell, both are interesting scenarios. $\endgroup$
    – Milo Bem
    Commented Jan 16, 2019 at 15:56

Theoretically yes. Practically no.

A pathogen say a virus can theoretically wipe out entire mammals if its deadly and there is no cure. All the mammals would get infected sooner or later and it would wipe out all of the mammals.

Practically evolution would kick in and one percent or .5 percent of the mammals would develop immunity to the pathogen by mutation/natural selection. Over decades these immune mammals would reproduce to produce more immune mammals and would repopulate the earth. All things equal and with the current scientific advancements this re-population would take decades rather than centuries.

  • $\begingroup$ Practically a small % of the mammals would already have a mutation that makes them tolerant or immune. That's how it starts. That clarification aside, totally agree with this answer (and voted it up). $\endgroup$
    – T.E.D.
    Commented Jan 17, 2019 at 19:30

About anything conceivable is possible. However, that scenario isn't particularly feasible.

In general immunologists will tell you that species only get entirely wiped out by a disease if that species already has a population problem, eg: its entire population is small and isolated in some way. That certainly can't be considered true for Mammals in general.


I don't think a lot of people have a good mental concept of how many mammals their are in the world, or how diverse and robust of an order mamalia is. In rodents alone there are over 2,000 different species, with habitats ranging over nearly every earth biome, and living on land, air, sea, and underground. This is a class of creatures that survived two of this planet's major extinction events, in which 80%, and then a further 76% of all living species were lost, including the Dinosaurs.

You are simply not taking out all of mamalia with a single cause, short of one that destroys the entire Earth. You could perhaps cull them back a bit, but you'd have to then explain why they didn't come roaring back.


Diseases themselves are rarely 100% fatal in their reservoir creatures for long, for obvious natural selection reasons: If all their host creatures die, the virus species will die too. So viruses themselves have a strong genetic pressure to become endemic rather than epidemic in their hosts.

Similarly, the body's immune system will eventually recognize and kill a disease (if it survives), which means a successful virus needs to not infect 100% of its host population at once, if it wants to survive.

When you tend to get really deadly outbreaks (eg: Ebola) is when a disease hops species from one its co-evolved to be endemic in to one that is naive. But even then, some individuals will survive, and have bodies that know how to survive this same disease in the future. It may take a few generations, but they'll be back.

In general, any time I see fictional stories claiming a 100% mortality rate for any disease, my arm reaches in my back pocket for the yellow BS flag.


While such pathogens cannot arise naturally via evolution as has been pointed out in many of the other answers, such pathogens can likely be made in the lab. Mammals likely have as of yet untested "zero day" vulnerabilities that have not been patched by evolution, because evolution cannot yield all theoretically possible pathogens. This therefore leaves open the possibility of creating such pathogens ab initio in the lab.

With today's technology, we can already create viruses ab initio and it doesn't require a huge amount of resources, see here:

Eradicating smallpox, one of the deadliest diseases in history, took humanity decades and cost billions of dollars. Bringing the scourge back would probably take a small scientific team with little specialized knowledge half a year and cost about $100,000.

In the future, a country like North Korea could produce new viruses that have never existed before. Such viruses can be evolved in the lab to make them ever more lethal. One can also imagine that 100% lethality cannot be achieved by a single pathogen, but there may then exist a solution that achieves this goal using two or more pathogens. Such binary bioweapons could also be developed to avoid early detection by making the individual pathogens give infected people only mild symptoms.

Then because such bioweapons gives countries the ability to win WWIII without all the problems associated with deploying nuclear weapons and trying to avert MAD, we have to assume that at least some countries are already developing such bioweapons. Such weapons may end up killing all mammals, so such a scenario is certainly not inconceivable.


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