system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Where do STDs come from if you can only get them from someone who has one?

Where did STI/STDs come from? I’ve always been taught that you can only contract ‘most’ of them from sexual contact. 

This begs the question of where did they originate from?
gpt
Most STDs came from close contact with animals. Not necessarily sexual contact, just close contact.

Gonorrhea came from close contact with cattle. A person working on an infected animal could easily be exposed to the bacteria. Milking a cow or helping birth a calf and then going to the bathroom without washing your hands would be enough.

Chlamydia is caused by a very common bacteria that affects all sorts of animals including humans. Most famously almost the entire population of Koalas are infected.

Syphilis may have come from Sheep or Cattle, but it also may have come from Llamas. Working theory is that it was a disease that early explorers brought back to Europe from America.

HIV likely came from a human eating an infected monkey or another primate.

Herpes is by far the most interesting (from a historical standpoint) because humans have always had Herpes. Our species inherited the disease from our evolutionary ancestors. Genetic testing shows junk DNA from Herpes infected our species hundreds of thousands of years before we even left Africa. Likely because one of our ancestors ate another infected primate. So Herpes, Warts, and Cold Sores are older than humans.

So when you hear on the news that we caught a bird flu, or a bat virus, it's because a disease mutated and jumped species to us. But does it work in reverse? Is there a common Human virus that could potentially jump to another species? yup... Herpes. 

Dogs can get Herpes Cold sores... and they got it from close contact with us.

**EDIT for clarity:** Dog Cold sores aren't like ours, but they get wart like growths and lesions and it can be very serious so if you suspect your dog is infected see a vet. 

Dogs have their own version of the Herpes virus and can't be infected by regular human herpes directly and vice versa. But Dog Herpes likely originated from humans ages ago when it jumped species and mutated.