system
Given the following post, respond as if you're commenting on Reddit:
human
Why and where do STDs come from? And why are they only transmitted sexually vs other forms of contact?
gpt
EVOLUTION, it's not just for the big species!  
STDs are viruses, bacteria or parasites that have adapted to live or spread around your genitals.  
Technically when you have Herpes or genital warts, your whole body has the disease.  However, you are most likely to spread it via small tears in your skin rubbing against small tears in someone else's skin.  Sex is one of only times that type of contact is likely to happen.  
Crabs are just a type of large lice that live in pubic hair.  Left untreated they will migrate to other warm moist hairy areas like your underarms.  Lice are commonly species specific (they evolved right along with us).  You cannot get head or pubic lice from a pet, only from another person.  
Not all infestations are bad.  Most of us have a symbiotic species of microscopic mites living in our eyelashes.  We are infested when our parents rub their faces against ours as babies.  There are also billions of bacteria in our intestines helping us digest food.