Disease Ecologies of Europe

felt that societies tended to have more males than females among adults because more women than men die between the ages of 25 and 45 (Russell 1977). This generalization, however, is based to a very great extent on the perceived epidemiology of tuberculosis, a disease neither universal nor of equal importance in all societies.

In Europe, several generalizations are usually accepted regarding human populations. Since classical times until the Industrial Revolution, cities have harbored more women than men. Cities have also been unable to replenish their own populations, at least from medieval times until the nineteenth century, because of the unfavorable disease climate of urban life (McNeill 1976; Russell 1977). Before the nineteenth century, the vast majority of humans in Europe lived in villages or on farms, with only a very small fraction residing in cities (Guillaume and Poussou 1970; Russell 1977). The total population of preindustrial societies contained at least one-third children, of whom less than half lived to adulthood from medieval until modern times (Flinn 1985).

Humans also affect their environment and in turn affect their own disease ecology. Practices we take for granted, such as the domestication of animals, change another species' disease environment, and the interaction of domesticated animals with humans has often led to human disease (Lovell 1957). An example of a disease that became of at least minor importance among humans primarily in this way is brucellosis, usually introduced to humans through goats. During the Crimean War, for example, Malta became an endemic area for brucellosis; in 1906 the means of human infection was described (Alausa 1983).

Disease in turn may alter the character of human populations. After major epidemics of infectious disease associated with high mortality, a period of markedly reduced mortality generally begins. It is felt that such epidemics "weed out," at least in some cases, the weaker elements in a population, leaving a healthier one behind (Wrigley 1962). On the other hand, diseases such as plague and some rickettsial infections often kill the healthy.

If we limit our generalizations about humans in Europe, our knowledge of their pathogens is even sketchier. The whole question of virulence among various bacteria is extremely complicated, and, since it is not well understood in modern infections and since organisms can mutate, virulence can be assessed only by effect. The effect of an organism in turn is related to host, pathogen, and environment, so that any comments about organisms in the past is unrealistic. Host-parasite relations vary from nonspecific, in which multiple species support the organism, to those in which the organism is dependent upon a single host. In the second case, it is felt that the organism has lost the ability to synthesize its own essential nutrients to an extreme degree (Lovell 1957). Leprosy is an example of an organism that cannot be cultured in an ordinary fashion and is highly host dependent (and even specific to certain cells in the host). This characteristic suggests that the susceptible tissues contain some chemical or provide some enzymatic function (or possibly something entirely different) that cannot be provided in culture media (Wilson 1957). If we consider virulence to be the property that permits an organism to cause death or damage to the body parts invaded, we are left with a multitude of factors to consider. An increase in virulence toward one species may decrease that strain's virulence to others (Wilson 1957), indicating the intimacy of the relationship to a given host. The same strain of Mycobacterium tuberculosis is generally much more virulent when entering the host via the respiratory route as opposed to the gastrointestinal tract, but with bovine tuberculosis the situation is the opposite. Such information, however, offers little practical guidance in the history of diseases, for the specificity of an organism toward humans (e.g., organisms causing leprosy, meningococcal meningitis, typhoid fever, and smallpox) only tells us that if the disease is present, so are humans, which is rarely in question.

In general, organisms are tissue specific. Clostridium tetani, for example, is harmless to intact skin, but can be deadly when in contact with abraided or necrotic tissue (Wilson 1957). The longer an organism is in contact with the environment and outside a host, the likelier it will be damaged. Organisms that affect the gastrointestinal tract, such as Salmonella typhi, are able to withstand environmental pressures longer than those not waterborne or foodborne (as a generalization) (Wilson 1957). Insect vectors may result in significant effects on the expression of disease. Arthropod vectors are more important, generally, in parasitic infections than in bacterial ones (Wilson 1957), but plague and typhus are notable exceptions.

Despite the staggering complexity of host-pathogen relationships, the presence of certain diseases can reveal important aspects of human populations. Measles is a human disease only and requires at least a population of some hundred thousands in some contact to remain endemic (Bailey 1954, 1957; Bartlett 1957,1960; Becker 1979; Smith 1983). Diseases such as measles and smallpox (another human disease whose pathogen needs a fairly large population to remain active) tend to become childhood diseases (McNeill 1976; Smith 1983). Thus the first observation of measles would tell us when a communicating population of humans reached a certain level. Unfortunately, we do not know when this occurred in Europe because we do not know when measles first appeared. The clinical expression of a given disease can also tell us about genetic factors in a population. Susceptibility to leprosy, in both its polar forms (although more in tuberculoid leprosy), is in part genetically mediated (Smith 1979; Eden et al. 1980; Keyu et al. 1985). When measles and TB have fulminant courses with high and dramatic mortality, they are present in a previously unexposed population (Wilson 1957; McNeill 1976). The same may have been the case when syphilis appeared in Europe at the end of the fifteenth century, but this is hardly an undisputed subject. It is also becoming apparent that some resistance to particular diseases reflects prior exposure to others that confer cross-immunity (Ell 1984a).

Characteristics of childhood diseases, especially measles, have become the subject of mathematical modeling. Birthrate has been shown to be an important factor in determining whether or not a disease like measles can remain endemic and in predicting interepidemic periods (Bailey 1957; Bartlett 1957; Becker 1979). Unfortunately, such models, although of great theoretical and some practical interest, cannot explain much of what occurs in a given epidemic. They are based on populations of constant size and without immigrants - factors from which deviation has been crucial in many other epidemics (Siegried 1960; Biraben 1975). This mathematical modeling is of very limited value in considering the disease ecology of Europe in the past. Too many of the factors are unknown (population size, birthrate), and the effect of social customs has not proved very susceptible to quantification. Further, in this chapter we perceive childhood mortality as a nearly constant background factor for much of the time period discussed, with major epidemics, which have proved resistant to neat equations, the more influential factors.

In the following pages, I try to discern, within what is known about the demographic history of Europe, the effects of disease on that history. Partly because war has been an invariable aspect of human behavior, and partly because it reflects so much else about a society, I focus often on military strategy, on the composition and size of armies, as well as on the direct effects of disease upon armies. The time frame considered extends from the hunter-gatherers of prehistoric

Europe to late twentieth-century society. I have very little to say about medicine, because I do not believe it has had a significant effect on the ecology of disease except in very limited circumstances.

Geographically, I define "Europe" as extending as far east as western Poland down through Greece, as far north as Scandinavia (but excluding Iceland), as far west as the British Isles, France, and Spain, and as far south as Greece, Italy, and Malta.

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