After graduating from Williams College (1955) and the University of Rochester School of Medicine (1960), Dr. Foster spent two years as a CDC Epidemic Intelligence(EIS) Officer assigned to the Indian Health Service in Arizona. In addition to his main responsibility for examining 10,000 school children per year for trachoma (25% were positive), he had the opportunity to investigate other health emergencies as they arose: Plague, Rabies, Measles, Shigella, Food Poisoning, KeratoConjuntivitis (Philadelphia, Talequah Oklahoma, and La Paz Bolivia), and Rotavirus in the Truck Islands in the South Pacific. In 1966, he was invited to join the CDCs new Smallpox Eradication Program. His family spent 4 years in Nigeria (1966-1970) and 4 years in Bangladesh (1972-1976) working with national health workers to eradicate smallpox. In 1977, he spent three months living with nomads in Somalia (the last smallpox epidemic country in the world.) In 1975 Dr. Foster confirmed and treated Rahima Banu, who was the last natural occurring case of severe smallpox (Variola Major) in the world.

From 1980 to 1994, Dr. Foster worked with the International Health Program Office at CDC in its Combating Childhood Communicable Disease Project (CCCD). He worked with 12 African countries to improve the health and survival of children under 5 through strengthening their capacity to prevent and treat diseases. Dr. Foster focused on prevention (immunization, malaria chemoprphylaxis of women); case management of the three priority killers of children (malaria, pneumonia, and diarrhea) and in strengthening their preventive and curative systems.