MEANS AND ENDS
The papers of these proceedings largely are examinations of some of the varied ways in which epidemiologic data may be collected from military and veteran populations and used in their study. Because these papers are about the means used to gather and analyze data, a word is in order about the ends that such activities serve. These papers have a wide applicability in that they are not only a part of military medicine but of clinical and scientific medicine generally. The study of military and veteran populations should be regarded as part of the study of the general population. The purpose of this introduction is to illustrate that the apparently specialized means employed in these studies are meant to serve the most general of ends: an understanding of the workings of disease in order to realize health.
The general applicability of results from studies of military and veteran cohorts is easily observed in the cases where such cohorts resemble other well-defined, usually occupational, cohorts. This is the case in the presentation by Brundage and the papers of Carmelli et al., and, to some extent, Jablon. Being infected by the human immunodeficiency virus (HIV), being a twin, or being exposed to low-level ionizing radiation are things that may befall members of the general population. Having made this obvious statement, one may ask the obvious question: Why study a military cohort?
There are at least two good, practical answers. First, the study of a military cohort can provide opportunities to gather information in a particularly efficient way. Second, military cohorts by their constitution often differ in small, yet important, ways from the nonmilitary cohorts available for study.
The first advantage in studies of military and veteran cohorts, efficiency of data gathering, comes about from things like the ready availability of material from which to assemble study cohorts and the possibility of attaining a very high degree of completeness of follow-up without great expense. For example, in the talk by Col. John Brundage,
which is not included in these proceedings, the fact that all active-duty Army personnel were required to be screened periodically for HIV infection makes available a large cohort of HIV-positive individuals. In addition, the annual examination of these individuals is already part of an ongoing Army program, so that follow-up information is obtained at low cost to the researcher, although the degree of completeness of this information is an issue that remains to be studied.
The second potential advantage of a military cohort is also illustrated by that study. The Army cohort is almost surely more representative of the U.S. general population than are, for example, the special cohorts, such as male homosexuals or hemophiliacs, whose study has provided most of the information available on the natural history of HIV infection to date. If nothing else, the Army's national data are not limited to a local geographic area. However, it should be pointed out that representativeness is a complicated matter, and that the active-duty military population is not strictly representative of the general population. In particular, the military induction examination screens from military service certain persons with selected medical conditions, no doubt with varying rates of success; hemophiliacs, for example, are almost certainly screened out. In addition, the military's proscription of homosexual activity and illicit drug use should, at least in the long run, tend to remove from military service many individuals engaging in these activities, which are associated with a high risk of acquiring HIV infection. Notwithstanding such reservations about the strict representativeness of the active-duty Army population, it is clear that much that is new will be learned by studying it and by comparing results derived from it with those obtained from other, even less representative, cohorts.
Moving from those studies in which a military cohort more closely resembles a nonmilitary cohort, we come to those studies in which a military cohort is defined by an event that would likely not be experienced outside the armed services. Such is the case for military captivity (Engdahl and Page), large-scale exposure of Americans to epidemic hemorrhagic fever (LeDuc et al.), or to herbicides such as Agent Orange (Kang et al.). A case for the general applicability of results from studHP LaserJet II DHPLAIID.PRSzed as relatively rare. Although some events might reasonably never be seen outside the military, others could occur in some fashion among civilian populations, although often the full range and intensity of the military experience is unmatchable. The study of cohorts defined by events and exposures that might reasonably occur outside the military is more easily recognized as being capable of producing results of general applicability. Yet even studies of cohorts whose experiences are truly unusual, and whose infrequency may at first suggest that their study will not produce any information relevant to general populations, may
nevertheless provide such information. Take, for example, perhaps the most singular case, the study of prisoners of war reported by Engdahl and Page.
As reported by these authors, the treatment of military personnel during captivity--as evidenced, for example, by an average body weight loss during captivity of almost 40 percent among prisoners of the Japanese--seems unparalleled in its brutality; thus, it might be expected to produce correspondingly singular psychological after-effects. However, it turns out that the psychological after-effects of military captivity are similar to the more general after-effects of other serious trauma, such as being the victim of a natural disaster. This similarity of after-effects was not immediately apparent and took a while to be discovered; thus, for a time, the psychological sequelae of military captivity were studied only in relative isolation. Now, however, it appears that the differences between the psychological sequelae of military captivity and other traumatic experiences may be differences of degree, not of kind, and that in studying adaptations to combat stress, one may learn much about psychological adaptation in general.
In conclusion, whether the military cohort is defined on the basis of a unique event or exposure, or on the basis of something largely ordinary, its study is meant to yield insights of a more general nature. This point needs to be kept in mind as one reads the papers in these proceedings, for while they possess something of a specialized nature, they have as their end the provision of information of general applicability. They demonstrate how the epidemiology of military and veteran populations is so rich an area of study.
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