How could a parasitic organism spread through humans?

Question

I have an idea for a crazy super-organism which could wipe out the human race. Essentially, it is an organism (plant or fungus or bacterium depending on what would be more effective or what would be possible) which spreads itself through the atmosphere.

Upon managing to find its way into the human body, it replicates itself. It may stay in the host for an indefinite length of time, or it may begin to poison the human in such a way that the human becomes unable to move (it could just kill them). Once the human is unable to move, the parasite begins to replicate itself on the outside of the body. It essentially uses the body to build a point of emission for its method of transmission, then spreads itself using the incapacitated human as a 'base camp'.

Is this possible, as far as biologists are aware? I've left a lot of things open here on purpose - the things which I need kept consistent are the method of transmission and the whole incapacitation thing.

Answer 1

Anthrax is very close to what you propose.

• It just kills you. Especially the inhaled type. It takes a day or two.

• The dead body of an animal killed by anthrax is the jumping off point for more infectious spores. It is recommended that animals which have died of anthrax are either buried deep or burned.

from http://www.fao.org/docrep/t0690e/t0690e0c.htm#unit 82: disposal of dead animals

Anthrax used to kill tens of thousands of humans and hundreds of thousands of animals every year. It is still a big problem in some parts of the world. Weaponized anthrax is a terrifying prospect.

Answer 2

If you are open to more wild ideas, why not a parasite that uses human hair as its vector? I mean, since we're constantly producing new hairs over our entire body, I can envisage a parasite that evolved to use hair to spread.

All people lose hair all the time. We literally produce a trail of arm, leg, chest, beard or head hair wherever we go. The parasite could have evolved in a way that it stays "dormant" on the host during years, reproducing like crazy to infect the host and producing spores or any other mean of propagation on hair follicles. Thus, each single strand of arm hair an infected person had left on a table or that was even carried by the wind is now an infection vector, that can spread the parasite to other human beings.

In average, a healthy adult loses around 100 hairs per day, counting just the ones from the head. For a normal, non-comically hirsute person, it isn't hard to think about a total of 500 hairs per day, between leg, arm, chest, back, beard... So, in a year this average-Joe loses almost 200,000 hairs. If the parasite stays dormant during a five year period, while it just uses the host to propagate through hair loss, we're talking about a million infected strains of hair.

As for the incapacitation, this could be another step on the infection. After staying dormant for years, just using the host to spread its hairs all over the world, the parasite awakens and start wreaking havoc on the person. With the amount of lost hairs, certainly thousands of others would be already infected, which in turn infected other thousands, so even in the moment patient-zero shows the first symptoms, it would be too late, because a very large number of people would be infected.

Answer 3

You need to consider a few things about the lifecycle of this pathogen as you have some contradictory aspects.

For starters, if the organism immobilizes the host prior to death, it is limiting how it can spread. Ideally a virus causes the host to move around, spreading the virus to other hosts. This is why virii that make us sneeze or cough can spread so quickly.

If it DOES immobilize and kill the host, then there really need to be an intermediate host that then comes and eats the host and carries on the organism. Say, for example, a fly that lands on the dead host to lay eggs. It gets infected by the host (or just carries spores on its legs) and then the fly lands on human food which is then eaten, infecting the human. This type of lifecycle would explain why an organism that paralyzes and kills could be transmitted from human to human (who normally are pretty smart about management of the dead and don't contaminate themselves with things that live on dead flesh).

Another option is to form fungal sacs that explode, spreading airborne spores in an area. But this will still limit transmission unless the infected host is first driven to go somewhere useful first before death, like water. Get a host to head to the nearest lake, stream or well, then die and spread water borne spores that can be ingested by humans downstream, that could work. Something that causes the throat to swell, preventing drinking, can cause dehydration and an overwhelming desire to try to drink. Obviously this works best in a primitive setting without faucets.