Epidemic Disease

An epidemic disease is an infectious illness that attacks a large number of people in a community within a short period, then subsides. Epidemics have shaped human history at least as profoundly as wars, religions, or political revolutions. They destroyed the Athenian empire, weakened Rome, devastated Native American civilizations upon European contact, and killed more soldiers in every major war before the twentieth century than combat itself. The shift from understanding epidemics as divine punishment to recognizing them as natural phenomena driven by living organisms is one of the longest and most consequential intellectual journeys in the history of medicine. That journey began with Hippocrates in the fifth century BC and was not completed until Koch and Pasteur in the 1880s.

Disease as Historical Force

Zinsser’s central argument — that infectious diseases, not generals, have been the primary drivers of human history — provides the sharpest framing of the subject. Swords and lances, arrows, machine guns, and high explosives have had far less power over national fates than the typhus louse, the plague flea, and the yellow-fever mosquito (Zinsser, 1935). War and conquest set the stage, but the herd existence that accompanies civilization creates the conditions under which parasitic organisms exert their real historical power (Zinsser, 1935).

The secondary consequences of epidemics have historically been more far-reaching and disorganizing than the mere numerical reduction of the population, as terror and ignorance led people to take actions that increased death rates (Zinsser, 1935). Modern bacteriology has brought about a state of affairs that may profoundly influence future economic and political history by converting some epidemic diseases from uncontrolled savagery to mild domestication and confining others to limited territories (Zinsser, 1935).

Ancient Epidemics

Bacterial diseases have attacked life since before the appearance of humans. Prehistoric animal remains show evidence of osteomyelitis, abscesses, and bone necrosis dating back millions of years (Zinsser, 1935). But epidemic disease — mass outbreaks killing thousands in short periods — requires population density. Greeks living as an outdoor people with no formidable population concentrations in early times suffered relatively few serious outbreaks; serious epidemics appeared only after large urban concentrations formed in classical Athens (Zinsser, 1935).

Hippocrates in the Epidemics provides case histories precise enough for modern diagnostic identification of the exact type of infection and often the responsible microorganism (Zinsser, 1935). Hippocrates understood nature as acting purposefully without conscious deliberation, and viewed healing as an expression of the life process itself, not as a separate healing power (Neuburger, 1943).

The Plague of Athens, described by Thucydides in 430 BC, is the first epidemic described in enough clinical detail to permit modern diagnostic speculation. It killed 300 knights, 45,000 citizens, and 10,000 freedmen and slaves, including Pericles himself, and laid low the power of Athens on land (Zinsser, 1935). Its exact identification remains disputed — smallpox, typhus, and bubonic plague have all been proposed — but its political consequences are not.

The Domestication Thesis

McNeill’s most influential argument concerns the “domestication” of epidemic disease. By 500 BC, each major civilized region of the Old World had developed its own distinct mix of endemic infectious diseases, creating separate disease pools (McNeill, 1976). Civilized societies possessed, without recognizing it, a biological weapon: their endemic childhood diseases devastated neighboring populations lacking immunity, facilitating territorial expansion more effectively than military force (McNeill, 1976).

This framework explains the catastrophic encounter between Old World diseases and New World populations. Civilized Amerindians by 1200 had developed dense populations without ever acquiring the endemic childhood diseases that Eurasian civilizations harbored, making them uniquely vulnerable to Old World infections (McNeill, 1976). The demographic result was staggering: North America’s pre-1492 population of 5-10 million fell to 600,000 by 1800 as smallpox, measles, plague, cholera, influenza, typhus, and yellow fever swept through populations with no immunological resistance (Jackson (ed.), 2011). Crosby documents the scale: in Mexico and Peru between 1520 and 1600, the record shows some fourteen epidemics in the former and perhaps seventeen in the latter (Alfred W. Crosby, 1972). Migration of man and his maladies, Crosby argues, is the chief cause of epidemics, and among major human divisions the American Indian had the dangerous privilege of longest isolation from the rest of mankind (Alfred W. Crosby, 1972).

Case mortality in an unvaccinated population runs approximately 30 percent; when smallpox first reached Iceland in 1707, 18,000 of the island’s 50,000 inhabitants died within two years (Alfred W. Crosby, 1972). In the Caribbean, smallpox arrived among the Indians of Santo Domingo in late 1518 or early 1519, killing one-third to one-half of the population; it touched few Spaniards, and none died (Alfred W. Crosby, 1972).

The “domestication” of epidemic disease between 1300 and 1700 — the shift from sporadic devastating epidemics to endemic childhood diseases — was itself a fundamental ecological event, resulting from the two great transportation revolutions of the age: one by land, initiated by the Mongols, and one by sea, initiated by Europeans (McNeill, 1976). The paradox is that the more frequently epidemics returned to a community, the less destructive they became, since they killed mainly infants rather than working adults. More-diseased communities were, counterintuitively, more demographically stable (McNeill, 1976).

The Synergy of War, Famine, and Pestilence

The three horsemen — war, famine, and pestilence — have never operated independently. War creates the conditions for epidemic disease: armies concentrate men in camps, displace civilian populations, destroy crops, and carry infections from one region to another. Famine weakens immune resistance and forces populations into closer contact as they crowd into relief centers. And epidemic disease kills more people than the war and famine combined.

Zinsser argues that it would have been impossible to permanently maintain a political and social organization of Rome’s magnitude without modern sanitary knowledge (Zinsser, 1935). Concentrations of large urban populations, free communication with Africa and the East, and extensive military mobilization inevitably produced uncontrollable epidemic outbreaks (Zinsser, 1935). Pareto failed to consider the calamitous epidemics that repeatedly swept the Roman world during its most turbulent political periods (Zinsser, 1935); among these was the Plague of Cyprian, which Cyprian himself described as killing 5,000 a day at its height in Rome and which struck for fifteen years (Zinsser, 1935).

The Plague of Justinian devastated the Byzantine Empire at the precise moment when Belisarius had reconquered much of the West (Zinsser, 1935). Procopius describes it killing 10,000 a day in Constantinople at its height (Zinsser, 1935).

Host-Parasite Coevolution

The evolutionary relationship between parasites and hosts follows a consistent pattern. Parasitism originates when one form of life adapts to the environmental conditions found in or upon another, beginning with a breakdown of normal vital resistance (Zinsser, 1935). As parasitic adaptation becomes more perfect over time, host reaction becomes less energetic and disease becomes less severe and more chronic, potentially reaching a stage of complete mutual adjustment where the host shows no signs of injury (Zinsser, 1935).

This principle explains the extraordinary lethality of “virgin soil” epidemics. When measles first came to the Fiji Islands in 1875, it killed 40,000 people in a population of about 150,000 (Zinsser, 1935). The first impact of a new pathogen on a fully susceptible population produces devastation far exceeding any subsequent epidemic. Conversely, scarlet fever became definitively milder throughout Western Europe, England, and America from about 1880 — a change that began well before modern preventive methods had any noticeable influence, illustrating natural epidemic attenuation (Zinsser, 1935).

This evolutionary framework has a disquieting implication. The louse, Zinsser warns, will never be completely exterminated, and as long as it exists the possibility of typhus epidemics remains. The disease is suppressed in clean, prosperous societies but persists in poverty-stricken populations and resurges whenever war, famine, or disaster create the conditions of lousiness (Zinsser, 1935).

The Shift from Miasma to Germ

The intellectual history of epidemic disease tracks the long transition from supernatural to naturalistic explanations. Hippocratic medicine replaced divine punishment with miasma — the theory that bad air arising from swamps, decaying matter, and environmental corruption caused epidemic disease. This was progress: it directed attention to environmental conditions and public sanitation. But it was wrong about the mechanism.

The 1832 cholera epidemic accounted for about 6% of deaths that year, ranking third behind consumption and convulsions (Bynum, 1994). John Snow’s 1854 survey showed that households receiving water from the Southwark company suffered over fourteen times the death rate of Lambeth company customers, and the interdigitating supply ruled out airborne miasmatic spread (Bynum, 1994).

The bacteriological revolution of the 1880s completed the explanatory framework. Koch’s isolation of specific causative organisms for anthrax, tuberculosis, and cholera, combined with Pasteur’s demonstration that microorganisms could be attenuated for use as vaccines, gave epidemic disease an explanatory mechanism, a diagnostic tool, and eventually a therapeutic response.

Medical Practice Before Bacteriology

Before the eighteenth century, the demographic impact of the medical profession was negligible (McNeill, 1976). McNeill argues that for every case where a doctor made a difference, others were harmed by treatments like bleeding (McNeill, 1976). The practical basis of the medical profession rested on psychology: everyone felt better when self-confident, expensive experts (McNeill, 1976).

Three global factors enabled worldwide population growth from the late seventeenth century onward, none of them medical in the conventional sense: the domestication of epidemic disease (as childhood diseases replaced sporadic catastrophic outbreaks), the spread of American food crops (maize, sweet potatoes, peanuts), and the consolidation of gunpowder empires reducing the frequency of warfare (McNeill, 1976).

Modern Containment and Its Limits

The bacteriological era converted epidemic disease from an existential threat to a manageable public health challenge — but not universally, and not permanently. Vaccination eliminated smallpox entirely. Water treatment and sanitation reduced cholera and typhoid to rarities in wealthy nations. Vector control campaigns suppressed malaria and yellow fever in many regions.

The attribution of epidemic decline to vaccination rather than sanitation was contested from within vitalist medicine. Henry Lindlahr argued in 1918 that the vaccine virus perpetuated smallpox rather than eliminating it by keeping the live agent in continuous circulation from host to host, and that every other epidemic disease — plague, the black death, leprosy, typhoid, scarlet fever, and diphtheria — had declined without vaccines, in proportion to the improvement of sanitary conditions.(Lindlahr, Henry, 1918) This argument anticipated by several decades the McKeown thesis, which used mortality statistics to argue that nutritional and environmental improvements, not medical interventions, explained most of the nineteenth-century decline in infectious disease deaths.

Charles Rosenberg observed in 1992 that with the expansion of interest in social and cultural history over the preceding quarter-century, epidemic disease had become an increasingly well-cultivated research area — the historian of disease no longer needed to feel defensive in demonstrating the relevance of their subject. The same could not be said, Rosenberg noted, of chronic illness, which remained comparatively neglected.(Jackson (ed.), 2011)

But the fundamental ecological conditions that produce epidemics — population density, poverty, war, and displacement — have not been eliminated. New pathogens continue to emerge from zoonotic reservoirs. And the evolutionary logic of host-parasite coevolution means that any relaxation of public health vigilance allows old enemies to resurge. Epidemic disease is not a problem that was solved in the nineteenth century. It is a permanent condition of human civilization, held in check by continuous effort and vulnerable to any disruption of the social order.

See Also

• Typhus
• Bacteriology
• Germ Theory
• Vaccination
• Public Health
• Plague of Athens
• Plague of Justinian
• Miasma Theory
• Vis Medicatrix Naturae
• The Columbian Exchange
• Host-Parasite Coevolution

Sources

All claims cite evidence cards from:

• Zinsser, H. (1935). Rats, Lice and History. Boston: Little, Brown. [Source ID: zinsser-rats-lice-history-1935] — Lead authority
• McNeill, W.H. (1976). Plagues and Peoples. New York: Anchor Press/Doubleday. [Source ID: mcneill-plagues-peoples-1976] — Lead authority
• Jackson, M. (ed.) (2011). The Oxford Handbook of the History of Medicine. Oxford: Oxford University Press. [Source ID: jackson-oxfordhandbook-2011]
• Bynum, W.F. (1994). Science and the Practice of Medicine in the Nineteenth Century. Cambridge: Cambridge University Press. [Source ID: bynum-sciencepractice-1994]
• Neuburger, M. (1943). The Doctrine of the Healing Power of Nature Throughout the Course of Time. Trans. L.J. Boyd. New York: J.J. Augustin. [Source ID: neuburger-healing-power-of-1943]
• Ackerknecht, E.H. (1955). A Short History of Medicine. New York: Ronald Press. [Source ID: ackerknecht-shorthistory-1955]
• Crosby, A.W. (1972). The Columbian Exchange: Biological and Cultural Consequences of 1492. Westport: Greenwood Press. [Source ID: crosby-columbianexchange-1972]