Biological factors that determine health may be classified as either endogenous or exogenous. Endogenous biological determinants include genetic heritage and immunity that is passively acquired from maternal antibodies passed across the placenta or transmitted in maternal milk. Genetic heritage is determined largely by responses to biological and physical environmental challenges to which human ancestors were exposed. These influence skin pigmentation and DNA composition that determines blood group, tendency to develop errors of metabolism, abnormalities of red blood cells (e.g., sickle cell trait), certain kinds of cancer, allergies, and many other conditions.
Exogenous biological determinants of health are the other living things with which humans interact. The most important are microorganisms; also significant are larger organisms that carry them. It is important to emphasize that humans could not live without microorganisms—they out-weigh all other forms of life on earth a hundredfold in biomass and are essential components in the web of life that makes up the global ecosystem. Many microorganisms are commensal or symbiotic with humans. The human body could not assimilate foods such as cheese if they had not been partially prepared before ingestion by bacterial fermentation, and it could not digest food without the help of bacteria that break down complex chemicals into simpler ingredients in our intestines. To maintain good health, people rely on relationships with innumerable microorganisms. Good bacteria are far more common than bad bacteria.
Only a small proportion of microorganisms are harmful. They are called pathogens because they cause pathological processes or diseases, and they cause most of the misery and suffering in the world. Encounters with harmful bacteria provoke defensive reactions; the immune system makes antibodies that confer partial or complete immunity. A person's immunologic defenses are influenced by his or her prior state of health, including nutritional status. Poorly nourished, protein-starved, and vitamin-deprived individuals are unable to mount effective immune defense mechanisms and are therefore more vulnerable to serious infection by invading pathogenic organisms. The potency or virulence of the organisms and their sheer numbers also affect their impact.
The European conquest of the Americas owes much to the fact that the Europeans had been exposed for generations to the pathogens that cause measles, smallpox, and tuberculosis; they therefore had some inherent immunity to infection. But the aboriginal populations had not, and so were devastated in epidemics that had extremely high mortality rates. In some respects, these aboriginal populations may not yet have fully adapted biologically to their changed circumstances. Before the European invasions, most had been hunter-gatherers, adapted by their evolution over hundreds of generations to a roving existence and a frugal diet. They were metabolically unprepared for a rapid dietary transformation, which helps account for their proneness to obesity and diabetes. The same phenomenon is observed in some Pacific Island populations.
The epidemic of coronary heart disease in the Western industrial nations in the 1940s that began to subside after the 1960s may have similarly been related to a failure of adaptation, that is, it may have had an evolutionary basis. The surge in coronary heart disease mortality and morbidity in Eastern Europe and the Indian subcontinent since the 1980s offers some support for this hypothesis, although other explanations, including a possible role for pathogenic organisms, have been suggested.
JOHN M. LAST
Ewald, P. W. (1994). Evolution of Infectious Diseases. New York: Oxford University Press.
Grifo, F., and Rosenthal, J., eds. (1997). Biodiversity and Human Health. Washington, DC: Island Press.