system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How do STDs infect human patient zero?
gpt
The diseases we describe as "STDs" generally aren't transmitted *only* through sex, but also through other sorts of contact with infected fluids and/or tissue.

Most of them probably entered the human population as a result of humans trying to eat an infected animal.  All it would take to catch something like HIV from an infected chimpanzee would be to have a cut on your hand (maybe because the chimpanzee is fighting back) and then splash the cut with some of its blood.