Fundamentally, prior to civilization there was not a sustainable host population for human diseases. Anything that could only target humans would quickly burn out as everyone around would either be dead or resistant from prior infection.
As our population grew and contact grew there were enough people around that a disease could sweep through a population then go elsewhere while a new generation grew up to be vulnerable when it finally came back.
Even with that, though, most of our disease are minor variants on stuff from the animal world. Covid appears to be simply the latest iteration of something that has periodically spilled into the human population in the past--except with the extremely nasty trait of asymptomatic spread. HIV is simply a minor variant on the already-existing SIV. It learned to target the immune system long before it targeted humans.
As our population grew and contact grew there were enough people around that a disease could sweep through a population then go elsewhere while a new generation grew up to be vulnerable when it finally came back.
Even with that, though, most of our disease are minor variants on stuff from the animal world. Covid appears to be simply the latest iteration of something that has periodically spilled into the human population in the past--except with the extremely nasty trait of asymptomatic spread. HIV is simply a minor variant on the already-existing SIV. It learned to target the immune system long before it targeted humans.