A comment someone made in one of the countless ID threads has really stuck fast.
I forgot who posited it, but the argument went something along the lines of “If the biblical account of creation is to be believed literally, all life forms on the planet were created all at the same time, in their present form. There are several parasites and disease-causing organisms which can only survive/reproduce in a human host. Ergo, Adam and Eve were totally clapped-out.”
I don’t want to re-ignite an ID debate, but I am curious: what parasites and other cooties could not exist without humans?
I haven’t gone through and cross-referenced all of these to see if they all require a human host for their lifecycle, but that is how they’re presented on the Ohio State University site where I got them all.
A few obvious candidates from the macroscopic world, before we get into the thousands of human-specific bacteria:
Body louse, Pediculus humanus humanus
Head louse, Pediculus humanus capitis
Crab louse, Phthirus punis
The mites Demodex folliculorum and D. brevis
The tapeworms Taenia saginata, T. asiatica and T. solium.
There are plenty of diseases to which humans become immune after the infection has run its course; starting from a population of two (or, for that matter, a population of eight in the ark), diseases like measles simply wouldn’t make it; they would run their course and become extinct because everybody would be immune before there were new human hosts on the scene to become infected.
Measles is not an obligatory human pathogen. It can infect several of the great apes at the very least. In reality measles itself is simply a slightly mutated form of a normal cow virus. Once again this doesn’t require any stretch to be incorporated into a scenario with only 2 humans 6000 years ago. Just as in relaity it could simply have jumped the species barrier sometime since then.
This is why the OP specifically asked for obligatory human parasites, not simply parasites with a single infective window. There was never any need for the first two humans to be acrrying measles, nor would the disiease necesrily be eradicated if the human population were reduced to two.
OK, perhaps measles itself was a bad example, but I’m sure there are pathogens (and indeed parasites) that would fall into this category of either killing or rendering immune the pool of hosts faster than they could reproduce.