A person can inherit AIDs in the blood they share with an infected parent. The child, while a virgin, can still pass the disease on to partners. How did the first person contract ghonorhea, syphilis, AIDS? It's an interesting question. Syphilis it turns out, was originally a virus found in sheep. So imagine a lonely shephard out with his flock...a little frustration, the sheep gives him a look, and BAM...syphilis comes into the human population. Now, they say AIDs started with small primates in the jungle...imagine a lonely researcher...out alone with his subjects...the monkey gives him the eye and....
All joking aside, infections come from a variety of sources. The disease could have been present during broken skin contact between an animal and human, or living in feces. So where the fist point of contact isn't human to human, once it's introduced and spreads, the virus replicates and mutates fairly quickly and spreads exponentially.