What conditions may have facilitated the jump of the virus responsible for AIDS to humans near Lake Victoria?

In The Hot Zone, Preston was told via an interview with some of those workers that the sick monkeys in Germany were not put down, but rather were shipped to a small island just off of Lake Victoria ( to be used as backups of the other monkey shipments were bad). The high concentration of infected primates, living in fairly close proximity to humans most likely gave the virus enough access to the human genome in order to make a change in its makeup.