As effective population size increases and as immunodeficiency increases due to the spread of HIV infection and an aging population, there will be more virulent organisms, predicts a new mathematical model devised by NYU professor Martin J. Blaser, a microbiologist renowned for his study of H. pylori and a mathematician, and Denise Kirschner of the University of Michigan.

As even larger societies developed, more virulent organisms, such as measles, emerged because the population could permit the virus to spread. Our most recent epidemics, including influenza in the early 20th century and AIDS today, involve organisms that can kill millions because these highly virulent organisms have a huge pool of people to infect, and still be transmitted.