Jtdcftul

Pathogen

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.

Pathogen

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

https://www.sciencedaily.com/terms/pathogen.htm

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.

Question

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?

Yes, but not in the way you are thinking.

One way this could happen is if some killer fungus starts growing unchecked throghout the planet, on land and sea. By killing all plant life it would destroy every other ecossystem, 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.

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.

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.

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.

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.

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. This construct is so deadly it would basically require humans to act with good intelligence to stop it at all, and even then one hemisphere appears doomed.

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.