June1966-September 1966 Louisiana State University - N.I.H. Fellow in Tropical Medicine
March1967 - June, 1967 H.E.W. Fellow in Pediatric Clinical Research, Jinnah Children's Hospital
July 1967-June 1968 Internship (Pediatric), Bronx Municipal Hospital Center, Albert Einstein College of Medicine
July 1968-June 1969 Assistant Pediatric Resident, Bronx Municipal Hospital Center, Albert Einstein College of Medicine
July 1969-June 1970 Senior Pediatric Resident, Bronx Municipal Hospital Center, Albert Einstein College of Medicine
January1970 - June 1970 Fellow in Pediatric Infectious Diseases, Division of Pediatric Infectious Diseases and Immunology, Department of Pediatrics, Albert Einstein College of Medicine
August 1970 - July 1972 Fellow, Division of Infectious Diseases, University of Maryland School of Medicine
July 1972 - June 1975 - Assistant Professor, Department of Medicine, University of Maryland School of Medicine
July 1975 - June 1981 - Associate Professor, Department of Medicine, University of Maryland School of Medicine
July 1981 - June 1982 - Research Professor, Department of Medicine, University of Maryland School of Medicine
July 1982 - Professor, Department of Medicine, University of Maryland School of Medicine
Jan. 1984 - Dec. 2014 - Head, Division of Geographic Medicine, Department of Medicine, University of Maryland School of Medicine
July 1971 - June 1972 - Instructor, Department of Pediatrics, University of Maryland School of Medicine
July 1972 - June 1982 - Assistant Professor, Department of Pediatrics, University of Maryland School of Medicine
July 1982 - July 1983 - Associate Professor, Department of Pediatrics, University of Maryland School of Medicine
July 1983 - Professor, Department of Pediatrics, University of Maryland School of Medicine
July 1984 - September 1985 - Acting Head, Division of Infectious Diseases, Department of Pediatrics, University of Maryland School of Medicine
October 1985 – July 2005 - Head, Division of Infectious Diseases and Tropical Pediatrics, Department of Pediatrics, University of Maryland School of Medicine
July 1974 - June 1978 - Assistant Professor, Department of Epidemiology and Preventive Medicine, University of Maryland School of Medicine
July 1978 - Associate Professor, Department of Epidemiology and Preventive Medicine, University of Maryland School of Medicine
July 1990 - Professor, Department of Epidemiology and Preventive Medicine, University of Maryland School of Medicine
July 1989 - Professor Department of Microbiology and Immunology, University of Maryland School of Medicine
1985 - Present Senior Associate, Department of Epidemiology, Johns Hopkins University School of Hygiene and Public Health
1980 - Present Visiting Professor, Facultad de Medicina, Universidad Peruana Cayetano Heredia
2001-Present, Visiting Professor, Faculty of Tropical Medicine, Mahidol University
Effective January 2015 Associate Dean for Global Health, Vaccinology and Infectious Diseases, University of Maryland School of Medicine
July 1974 – Dec. 2014 Director, Center for Vaccine Development, University of Maryland School of Medicine
1987 – 1990 Acting Co-Director, Medical Biotechnology Center, University of Maryland