Microorganisms like bacteria and viruses that cause STIs can interact with the animals and people around them. Sometimes bacteria growing inside you can be really harmful like the bacteria that gives you food poisoning! This is exactly what an STI is like. STI’s are infections by microorganisms that are harmful for our body.
The first person who got one probably got it by chance. Its really hard to say how they caught it…. It was probably a microorganism that infected another part of the body and it traveled there either by its self or through sex and then was able to grow easily in that part of the body causing infection. It could then have passed from person to person.
Monkeys, a bottle of red, and mood lighting (Ken…on the origins of AIDS)
Also some other suggestions- people are covered in microorganisms- actually, if you count the total number of cells in our body, then compared that to the number of microorganisms living on or in us- then technically we are 90% bacteria. Mutations in the DNA of bacteria (or organism of choice) may be responsible for the start of an STI. In this sense, for the human who originally had it, the mutated organisms may have been in check by their immune system. However, through sex, this bacteria transferred to another person, of which, whose immune system could not control it. Add some free love to the equation= STI.