As one historian notes in his account of the Vietnam conflict, "there was no 'typical' U.S. soldier in Vietnam … the three million Americans who served there went through many varied experiences—partly because the quality of the war varied in different areas of the country, and partly because the nature changed over time" (Karnow, 1991). Individual experiences also varied according to job assignment, military unit of service, rank, and branch of service. Artillery units, for example, tended to be less mobile than cavalry because of the heavy equipment involved. An individual assigned to base headquarters with an Army personnel position experienced a different tour of duty than an infantry commander, a field engineer, or an officer stationed aboard a Navy vessel off the coast of I Corps. Personnel assigned to units in the Mekong Delta might slog week after week across paddy fields, while others patrolling the perimeters of major U.S. installations at Danang, Bien Hoa, and Camranh were often targets for sniper attacks (Karnow, 1991). Individuals and units also varied in their consumption of locally grown foods and water from local supplies, as well as in their personal hygiene practices. Ground forces—the Army and Marines—were likely to experience more of the day-to-day fighting than Navy or Air Force personnel (Card, 1983). Sociological assessments of the American soldier in Vietnam suggest that no one factor is more important in understanding the experiences of the individual veteran than the degree of exposure to combat (Moskos, 1975; Fischer et al., 1980; Martin, 1986; Shafer, 1990).
In order to properly evaluate existing epidemiologic studies of Vietnam veterans and to consider the possibility of new studies, the size and characteristics of the exposed population must be known. Remarkably, the number of U.S. military personnel who served in Vietnam during the Vietnam conflict is not known precisely. Estimates depend on definitions regarding time and place of service, and the source of the data on which the estimates are based. Although detailed records of demographic information were not compiled during the Vietnam era, some federal estimates are available. In addition, data from several national surveys of the Vietnam veteran population supplement the government estimates.
According to official records, U.S. military advisory assistance to Vietnam began as early as 1950, during the First French-Indochina War; 128 personnel "spaces" were allotted for the U.S. advisory group (MACV, 1972). After the division of Vietnam along the seventeenth parallel in 1954, U.S.