The low incidence of some infectious diseases reflects effective public health prevention measures, such as vaccinations. For a select group of zoonotic infectious diseases with high death rates, the low incidence reflects infrequent spillover from an animal reservoir into humans. Often, humans represent a dead-end host for these pathogens, and person-to-person transmission is rare if appropriate infection control practices are followed.

see the cdc paper at http://wwwnc.cdc.gov/eid/article/20/2/13-1748_article.htm