Measles

“Measles is a density-dependent disease that requires a population of about 300,000 to maintain itself and was unknown in many rural areas of the world until they were “invaded” by town dwellers.”

“Of the density-dependent diseases, measles requires a population of about half a million to become endemic, and the others probably less; in other words, while no city in Greece could support smallpox, it could certainly have become established in Rome.”