Not a surprise at all, but a pleasant one nonetheless.

From the BBC:

HIV is evolving to become less deadly and less infectious, according to a major scientific study. The team at the University of Oxford shows the virus is being "watered down" as it adapts to our immune systems.

It said it was taking longer for HIV infection to cause Aids and that the changes in the virus may help efforts to contain the pandemic. Some virologists suggest the virus may eventually become "almost harmless" as it continues to evolve.

This is something else covered in "Guns, Germs and Steel". We have observed similar things happening with many other infectious diseases for example syphilis.

Basically, germs want to maximise their own number. So the ideal outcome is for their hosts to remain alive but infectious.

There is no mileage in killing off your host; germs like it when their hosts produce and discharge more germs to infect others (coughing, sneezing, vomiting and other things too disgusting to mention on a family blog). The unfortunate side-effect of this, from the germs' point of view, is that they overdo it and the symptoms kill their hosts (who can no longer infect others) and thereby some of their own number.

The side effect of becoming less lethal is that they become less infectious as well; there's simply not the hurry. If you are a disease that kills within a couple of days, each host has to infect at least one other host in the next couple of days. If you are a slow-killer or non-killer disease, you can take your time, if your host only infects one new host every few years, then that is enough.

Diseases which kill nearly everybody they infect soon disappear because they run out of victims; either they are dead or immune (by luck or acquired). We see the same thing happening with Ebola, there's a nasty outbreak every decade or two, then it all quietens down again.

See also: why animals on isolated islands tend towards vegetarianism.