Infectious diseases are out of whack
A little while ago I finished reading Plagues Upon the Earth: Disease and the Course of Human History . It was a a very good book about exactly what it says on the tin. There was a bunch of fascinating science in it but one part clarified some ideas I'd had noodling around in my head prompted by Covid and I'm going to outline those bits here. Back in prehistory there were very few epidemic diseases spread from human to human. In a band of 100 people a disease like influenza will rip through everyone in short order. But afterwards, everybody's immunity will be at a peak and that disease lineage would die out. This isn't to say that humans would never get the flu, just that each outbreak would be the result of a new zoonosis or crossover from animals to humans. Other diseases like Cholera exist harmlessly in the environment most of the time, attaching to the shells of crabs, until some human happens to drink it in and then it expands out of control with us as just