OK, I realize that out of the millions of bacteria and virri out there, oly a small number are harmful to humans. My question is: how does an EXISTING microorganism decide to prey on humans? Take the AIDS virus-current theories suppose that it started out as a monkey virus, and somehow began to colonize human hosts, at some time in the 1950’s. I understand about eveolution, but in a practical sense, the virus doesn’t “know” that there are humans “out there”-so how does it “learn” how to penetrate the spinal cord, and migrate to the brain? And, since it is a parasite, why does it kill its host? Seems to me it would prefer keeping us alive-after all it has a nice warm body to live in!
Second question: we now have this huge incidence of “foot and mouth” disease in our domestic cattle, shhep and goats. How long before this organism jumps to humans, and decides that WE are suitable hosts for it?
Well, as far as the first question goes
I believe that biologically, monkeys are very similar to humans. So, it doesn’t seem outrageous to hypothesize that a monkey virus wouldn’t have to change all that much to turn into a human virus.
Also, viruses mutate all the time, so it doesn’t seem outrageous to guess that a monkey virus mutated the right way, in the right place, at the right time.
As far as the second question goes - why does a virus kill its host – it’s worth noting that many (most?) viruses in fact don’t kill their hosts.
And viruses that do kill their hosts, often don’t get far. Consider Ebola.
Of course, the virus is always going to be at least a little dangerous to its host, since it’s coopting its hosts resources for its own use.
And my understanding is that many viruses evolve into less deadly forms, since the less deadly forms are able to propogate further.
Thus, my WAG as to why AIDS kills is that it is so new. Given enough time, it would adapt to humans (and humans would adapt to AIDS). End result - a “better” AIDS.
P.S. Richard Dawkins has a great discussion of this issue - lethality of viruses - in one of his books. I believe it’s in “The Selfish Gene.” I highly recommend this book to you.
The latest thinking is that HIV 1 migrated from a subspecies of West African chimpanzee to humans. Another form of HIV, HIV 2, is linked to a simian immunodeficiency virus (SIV) from a monkey called the sooty mangabey.
[Edited by JillGat on 03-30-2001 at 04:35 PM]
You’re thinking backwards. It’s not that the virus finds itself in a human and decides to adapt to its new situation. In any infected chimp, there would be a variety of mutants that differ slightly. A mutant would spontaneously arise that would be better adapted to humans. Because this would probably make it less adapted to the chimp, it wouldn’t spread, but it could get lucky and get exposed to a human.
On the flip side, humans may have been exposed to the virus several times without encountering one that happened to be well adapted to the new environment. In this case, no infection would occur, so we’d never even know.
To address one of your questions, the reason HIV doesn’t mind killing the nice warm host is that it’s only in the host in the first place to reproduce; it’s not worried about setting up a comfy life with a couple cars and 2.5 kids.
In the case of HIV, maybe a smart adaptation would be to modify itself so that it increases the host’s sex drive. I wouldn’t hold your breath, though. 
FD.
Viruses do evolve lower lethality, just as the hosts evolve resistance.
The typical pattern for a disease is that it starts as a benign infection in a host species…the host doesn’t get sick, the virus replicates, everyone is happy. Then somehow a member of another species is exposed to the virus, through any of the many well known ways that viruses can be transmitted from one individual to another.
That virus is not adapted to it’s new environment, and the new host is not adapted to the virus. The host gets sick, and perhaps dies. If the average host dies before infecting at least one other person, then the virus won’t be able to establish itself in the new host. Eventually, by chance, a virus that is slightly less lethal to the host evolves. The host passes the virus to many other hosts, before they die…and now the virus has potential. It breeds rapidly, and the high population means that there is more opportunity for genetic variation. So, ever-less virulent strains of the virus appear.
And all the while, the host population is evolving too. Indidividuals who are very susceptable to the virus die. Individuals who are resistant live, and have more offspring. When the virus becomes less lethal and becomes an epidemic, then the host population really starts evolving, since when the virus was isolated there wasn’t much of a reproductive advantage to being resistant.
So, the virus and host co-evolve into a state where the virus doesn’t do much harm…until the virus jumps to another species.
We see this sort of thing with Feline Immunodeficiency Virus. FIV is pretty much equivalent to HIV, and is lethal for housecats. But there are populations of wild cats…lions, leopards, etc…that are 100% FIV positive. But they don’t die. They have evolved resistance, and the FIV strains they are infected with have evolved to be benign. In another couple of thousand years, FIV will no longer be a problem for housecats.
And HIV would eventually do the same for humans…but only after millions of deaths and thousands of years.
Why do parasites sometimes kill the hosts? Because once the original parasite has reproduced and spread its genes, none of the individuals matter in the least. As long as the genes get passed along and copied, the species will survive. Maybe eventually a better strain will evolve, but that’s hit-and-miss until the ball gets rolling in that direction, with various random changes adding up into something. Then it can happen very quickly, in only a few generations. AIDS hasn’t hit upon that yet, and it might not for some time now, so our best chance is to develop effective medicines to fight it.