1974-1975: Research Assistant, Johns Hopkins School of Hygiene and Public Health, Baltimore, MD. Member, National Geographic Nepal Primate Expedition, Kathmandu, Nepal.
1975-1976: Fellow, Thomas J. Watson Foundation (Canada, Nepal, New Zealand, Peru).
1976-1977: Teaching Assistant, Department of Biology, U. of Pennsylvania, Philadelphia, PA.
1978-1983: Fellow, NIH Medical Scientist Training Program, U. of Pennsylvania.
1985-1988: Resident, Internal Medicine (Primary Care), Massachusetts General Hospital, Boston, MA.
1988-1991: Fellow in Infectious Disease, John Hopkins Hospital, Baltimore, MD.
1992-1998: Director, Malaria Program and Head, Department of Clinical Studies, U.S. Naval Medical Research Unit #2 (US NAMRU-2), Jakarta, Indonesia.
1998-2004: Director of Clinical Research, Malaria Program, Naval Medical Research Center (NMRC), Silver Spring, MD.
2004-2011: Head Malaria Department, NMRC.
2004-2013: Director, US Navy Malaria Program, NMRC.
2004-present: Adjunct Assistant Professor, Uniformed Services University of the Health Sciences, Bethesda, MD.
2010-2013: Research Coordinator, U.S. Military Malaria Vaccine Program, NMRC, Walter Reed Army Institute of Research (WRAIR), Silver Spring, MD.
2013-2014: Vaccine Consultant, Thomas L Richie Consulting, NMRC, World Health Organization, Geneva
2014-present: Chief Medical Officer, Sanaria Inc., Rockville, MD
2016-present: Director of Vaccine Discovery and Clinical Development, Sanaria Institute for Global Health and Tropical Medicine (SIGHTM)