system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How did STDs and STIs get started?

Was it some sort of mutation within a population or a species?
gpt
STDs are viruses of the plant and animal kingdom that prove problematic (to say the least) when spread to new hosts.  HIV as we all know was an infectious disease in monkeys that spread through handling their blood during food preparation.  Herpes was a virus in sheep that spread.  When Lewis and Clark voyaged across America they contracted stds from Native American women which they showed no signs of--simply because they had adapted to them.  When these viruses infect a new host that has no antibodies (or w/e) to them, the signs show up.  This is why some people can be carriers without symptoms while others get itchy dick.  It's very scientific.