Studies on measles outbreaks in island populations of varying sizes, such as Iceland, Greenland, Fiji, and Hawaii, show that a population of around 500,000 is required for the virus to circulate continuously in a community, a figure that is probably similar for other airborne viruses. The first towns of this size evolved around 5000 bc in the Fertile Crescent, and so sometime after this date, viruses like measles were able to break the link with their animal hosts to become entirely human pathogens.
