I love how that got down voted and the general amount of HIV fud in this thread - I actually do HIV research, I would know.
Most HIV people get infected by one strain, but it mutates so rapidly that in a single individual, multiple strains co exist at different levels (a major one and many, many lower levels ones).
I doubt that infection with one strain of HIV is preventing infection with another strain, except to the extent that some of the people engaging in the most risky behavior are getting "taken out of the system" by AIDS. Something like this could lead to exponential decline among certain subnetworks of society.