Tackling the complex causes of epidemics, Dr. Donald Ganem explains how mutations in genes and changes in the environment and human social behavior can give rise to new infectious diseases. He cites the influenza virus as an example of genetic changes that have led to epidemics and pandemics. He also shows the impact of weather on a 1993 outbreak of Hanta virus, describes the effect of human migration on the spread of smallpox, and examines what happened when the myxoma virus was introduced in Australia in the 1950s to control the rabbit population. From the Series:HHMI’s Understanding Biomedical Research Series