Actually, this is probably similar to what biologists have observed with other diseases.
If the disease has relatively few chances to spread, it will lie low and slow down its maturity cycle so that it can spread to as many people as possible before it does its damage. If it matures too fast it kills off all the hosts too quickly and won't be able to sustain itself.
However if the disease has relatively many chances to spread, it will mature as fast as possible so that it can make as many copies of itself as possible. It doesn't matter that the hosts die quickly, because it spreads fast enough to overcome that.
What this probably means is that there are so many instances of unprotected casual sex and sharing of drug needles that AIDS is getting more and more chances to spread.
