What are the limiting factors of pathogen population size in human populations?
One of the limiting factors is how many uninfected hosts exist. As a pathogen becomes more widespread, the probability of someone infected with it infecting someone else decreases, as it's more likely that the person will spread the disease to someone who already has it instead of someone who doesn't have it, preventing the virus's population size from increasing. See this for more information.

Also the more common a pathogen is, the more likely it is that people will take actions against it, for example by being more sanitary or getting vaccinated against it.