Crowd Diseases

"Crowd diseases" are diseases the flourish in crowds or large, centralized populations of humans.  In the last millennium, smallpox, cholera, tuberculosis, plague, diphtheria, and flu  produced epidemics throughout Europe because of the geography, climate, and proximity of the people.  These diseases found a foothold in populations that lived closely together, particularly through winters where life was spent mostly indoors.  

In addition, many of these crowd diseases began as diseases that took hold in the animals Europeans had domesticated and with which they had close contact.  Over several generations, these "domestic animal" diseases mutated to infect the nearby human populations.

Two things helped human populations in Europe survive these diseases.  First, since humans in these areas worked closely with domesticated animals, many developed some immunity to the human form of the disease through contact with the form that infected livestock.  Milkmaids, for example,  who contracted cowpox from the livestock did not contract smallpox, the more deadly human form of the same disease.  In addition,  European populations of the 1500's and 1600's had many generations to select persons with genes conferring some immunity to these diseases.  These two factors eventually led to a naturally immunized core population.  

When European explorers came into contact with the indigenous peoples of the Americas, the microbes the Europeans were carrying and had developed immunities to, decimated native populations that, because of different circumstances, had not developed these natural immunities.