system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Since STDs like gonnorhea can only be sexually transmitted, how did they get on the genitals in the first place?
gpt
The same way any disease comes to infect a human: A microorganism adapts and evolves in such a way to allow it, and it succeeds enough to further develop into a distinct species. 

Considering that there are 10x as many microorganisms in a person's body than there are their own cells, there would be a lot of chance for this to happen.