- 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.
- Neville Morley (19 December 2002). Metropolis and Hinterland: The City of Rome and the Italian Economy, 200 BC-AD 200. Cambridge University Press. p. 43. ISBN 978-0-521-89331-2.