How did the mixture of monkey Samples and species contribute to the hypothesis about HIV being a hybridized or mixed-origin virus?

Because of the way they were housed, different species of monkeys were thought to be passing infections and rare diseases among themselves. Poor conditions (hygiene, protection) could easily have made it possible for some of these conditions to have been passed onto the people who worked with them. This led to the hypothesis that hybridization may have contributed to the origin of HIV.