Would it be possible that other planets, or even other random celestial bodies, contain different types of pathogens? According to Wikipedia, the known types of pathogens are: virus, bacterium, prion, a fungus, algal or eukaryotic organisms. So, is the universe limited to these types of pathogens, or is it possible that somewhere out there, there exist another, or multiple other types of, pathogens? If so, how many types could there be?

If nature fails to deliver, could a hypothetical, very advanced, civilisation create new types of pathogens?

The goal is to affect life on earth, by for example, storing the pathogen inside a comet that is on a collision course with earth. Other suggestions concerning the delivery method are welcome as well.


There could be as many pathogens as there are types of self-replicating organisms or molecules.

Bacteria and eukaryotes are earth organisms. Other planets would likely have radically different organisms, any of which could be a pathogen. Both single-celled and multi-celled organisms can act as pathogens here on Earth, so alien single-celled and multi-celled organisms (or even organisms that lack anything we could identify as a cell) could be pathogens.

It wouldn't have to be a full organism, any self-replicating molecule could be a pathogen, as long as it can successfully enter the body and find the raw materials to replicate inside. This could include molecules not found on Earth. It can also include molecules that are found on Earth that have the potential to self-replicate, but don't currently self-replicate on Earth, such as RNA or DNA (there are "naked" RNA pathogens, viroids, but they use a cell's replication machinery rather than replicating themselves, and in the cell RNA and DNA need other molecules to replicate). Similarly, prions work by causing other proteins to mis-fold into more prions. But RNA and DNA also fold in important ways, so there is a potential for RNA or DNA equivalents of prions to exist (although as far as I am aware none are known).

Finally, from what I have found there is one (and only one) group of organisms on Earth for which no known pathogenic forms exist: archae. However, archae are particularly hardy, often living in extreme environments. This could make pathogenic archae hard to defeat with our natural defenses or medicine.

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    $\begingroup$ Good answer! Kind of scary though... ;-) $\endgroup$ – JBiggs Mar 9 '17 at 18:26
  • $\begingroup$ Very informative and also the answer I was secretly hoping for, because it opens up creative possibilities and makes the worldbuilding more believable. Thank you! $\endgroup$ – Dirgeful Zero Mar 10 '17 at 10:34

Don't forget all the large (visible sizes) parasites.

Some pathogens are inadvertent. E.g. Botulism and typhoid both kill accidentally -- they produce a chemical that is toxic to their host.

Your aliens could drop a mold on the earth that ate grass, and produced ricin (the poison in castor beans) as a byproduct.

  • $\begingroup$ I like the idea about the pathogen being inadverently deadly, didn't think about that one yet. $\endgroup$ – Dirgeful Zero Mar 10 '17 at 10:36

Different answer because different concept:

Bugs can also be anti-technology, rather than anti-personnel.

The puppeteers destroyed the Ringworld civilization by introducing a bug that ate super-conductor plastic.

There was a series in Analog some years ago about 'the oil bug' that turned oil into black crud. Didn't care if was refined into gasoline or diesel first.

Consider the effect of a bug that got it's main energy from 4Fe +3O2 => 2Fe2O3 Turning every exposed piece of iron into flakes of rust.

Another wayward thought is to make them anti-ecology. A virus that needed magnesium, and would take it out of chlorophyll. A bug, designed as an anti-climate change agent that fixes carbon dioxide as calcium carbonate -- And it works too well, dropping CO2 down to 100 ppm, and the planet freezes, as plants do poorly due to lowered CO2 levels.


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