| You have a couple of bad assumptions.
Other species do get STDs but they don't live long enough or have sex often enough for them to spread far. Most wild population have limited mating periods and the population groups are separated.
When you are looking at humans you have a high population tightly packed and very connected.
When a disease does appear it can spread fast and far. Even if the disease is lethgal in a fairly short period it can still be spread fast just because of how many people we are in contact with. In a wild population such a disease would only kill off the carrier or a small group before running out of victims |