I find that while not strictly speaking accurate, it helps to think of AIDS and bubonic plage as taking advantage of the same..."security hole" in our immune systems, while otherwise being very different from each other. The mutation patches the hole, probably at the cost of some other tradeoff, which is much less important in an epidemic.
It's well worth noting that HIV/AIDS isn't that contagious if you're engaging in heterosexual, PIV sex. The odds are about 1 in 500, even without a condom (we're not talking astronomical odds here, though, so nobody run out to have unprotected sex with someone infected with HIV). Vaginas are usually well-lubricated, so there's less chance of tearing and therefore, viral transmission.
Yes...but. Not only does that make for bad cumulative odds in a sexually active relationship but: The odds are better than that for males, and twice as bad for females, who recieves infected bodily fluids. The odds are far worse, especially for females, if the encounter is violent. The odds are also worse if the participants have infections, sores or rifts on the genitals.
Would immunity from HIV spread? Due to nature of the virus it doesn't really give you an edge. People can be infected and pass their non-immune genes on before dying.
Unless immunity works in such way that immune + non-immune parents always produce immune children. With 2,3+ children birthrate this could work but, as I said, it would require immunity gene to always cancell non-immune one.
In addition to what Mipp said...an infected mother has about a 30 % chance of passing the infection on to the child during pregnancy and childbirth. Afterwards...well breastmilk carries HIV. So a child with non-immune parents are unlikly to live to sexual marturity.
Also, with an average incubation time of 8 years, infected parents are likly to die before the offspring is old enough to care for itself. Which is often fatal in a medieval setting. However, there is a chance many children would be taken in by extended family. AIDS being difficult to recognize as a disease works in the childs favor. In general, however, losing one or both parents at a very young age will seriously impair your chances of survival in a medieval setting.
What would happen with a virulent version of AIDS is that most people who are nullizygous (that is, not having a copy of that gene) would die out pretty quickly, leaving only the homozygous and heterozygous individuals. Those would then mate amongst one another, and within a couple generations just about everyone would have at least one copy of CCR5 delta 32.
I suspect herd immunity would kick in at some point here. But I have wondered about something: How big is the advantage of resistance in an environment with no antivirals or medical care? Does it allow you to resist infection, or will it just mean you end up in the top end of the 3-20 year latency period?
Asking a question out of ignorance here. But it just occurred to me that the way HIV/AIDS works looks a lot like a disease that has evolved or partially evolved to adapt to its human hosts. The long latency period when the carrier can still be contagious in particular.
Has there every been any thought that HIV might have originally been a deadlier disease with more immediate effects that has since been 'toned down'?
Its probably co-evolved with its original, simian hosts. Humans just lack the adaptions that would let us eliminate it during the long latency.
