After all, viruses, like other forms of life, should in theory evolve to maximize their proliferation given the constraints of their environment. If one constraint is that opportunities to leap from organism to organism don't arise very often—if there's little promiscuity and little drug use, say—then the viruses can't "afford" to kill their hosts very quickly; viruses that killed rapidly would themselves die without reproducing. If, on the other hand, opportunities to leap from host to host do arise often—if people are wildly promiscuous, or are sharing hypodermic needles frequently—then the virus can evolve high lethality; it can kill off its host in a few years without extinguishing itself, since by then it may have transmitted lots of copies of itself into posterity via other hosts. Thus, in the opinion of some evolutionary biologists, did sexual promiscuity and frequent needle sharing turn the AIDS virus into a speedy killer.