The reasons that epidemics occur largely are a function of human behaviors and responses to environmental changes, known as the social ecology of infectious diseases. The AIDS epidemic began in remote parts of Central and West Africa and spread from rural areas to urban centers. The epidemic became a global pandemic. Although screening programs and use of antivirals have decreased the likelihood of HIV transmission, these advances have not reached all centers in the developing world. The epidemic continues to grow despite effective treatment because of the absence of a protective vaccine and the continuation of high risk sexual behavior and substance use.