I wish they would talk more about how the jump was made. Was it from eating chimps, getting bitten by them, or using them in perverted sexual stunts?
That may not be known. However, given the frequency of activity and the amount of bodily fluid exposed, butchering and eating seems most likely.
I would think the "perverted sexual stunts" was how AIDS was passed. Someone got WAAYYY too close to the chimps. 'Course maybe they just ate wrong portions of chimp body. (gross either way)
chop the raw liver out of a chimp, grow smallpox vaccine on it, filter it, and squirt it into the mouths of millions of black Africans wanting to be immune from smallpox. Some of them are going to have cancer sores on their gums...
My first guess would be while butchering them, or eating improperly cooked meat.
Mark