Smaller insular groups get wiped out by infectious diseases, but they don't communicate them to other insular groups. And that is the going hypothesis of why we really like to organize in small bands about the size of our Facebook friends list, and want to throw rocks at people from other tribes.

Larger intermixing populations gain their immunity by first having a lot of them die off. This is one of the benefits(?) of ranching livestock, since they're a vector for diseases that commonly get transmitted to their human owners. The epidemics happen, and then the survivors have the antibodies, and even pass them on to their kin.

Nowadays, we have centralized disease control. It may be failing in the US but the rest of the developed world still have a robust system. But human beings haven't had time to evolve (very much) towards large pluralistic societies that combat disease through science and proactive prevention.

Given enough time, I'd think we might, since large pluralistic societies also develop science, technology and arts much faster in general, often much to the chagrin of moral guardians like religious ministries.