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By Root 404 0

horrific epidemics. Major smallpox epidemics arose in two distinct epidemiological situations. In a so-called virgin soil population, one that had never been afflicted with smallpox or had been spared the virus for many years, a single epidemic could be devastating. In 1241, the people of Iceland had such an encounter with variola: some twenty thousand of the island’s seventy thousand people died. The experience of indigenous populations of the Americas with epidemics of smallpox after the arrival of the Europeans in 1492 is well known if not easily fathomed. Many factors may have contributed to the extraordinarily high susceptibility of sixteenth-century American Indians to smallpox, including malnutrition, dislocation, and poverty—problems caused or exacerbated by the violent process of European colonization. But the likelihood that American Indians and their ancestors had no previous contact with the disease helps explain mortality rates that ran from 50 to 80 percent. Variola was the deadliest killer in a terrible onslaught of alien microorganisms that, by some historical estimates, may have decimated as much as 90 percent of the precontact population of the Americas.25

A different sort of epidemic occurred in well-populated places where smallpox was more or less always present, such as parts of late eighteenth-century Europe and England. The number of susceptible individuals in a community gradually built up over time, creating fodder for an “epidemic year,” when smallpox became suddenly widespread and lethal. In this situation, where a majority of the adult population, including most of the breadwinners, was immune from previous infection, an epidemic could cause untold misery without seriously threatening the population’s subsistence. 26

As with many infectious diseases, the incidence of smallpox rose and fell with the seasons. Climate, social factors, and the traits of the virus itself conspired to make smallpox a disease of the winter and spring. Variola remained viable longer at cooler temperatures. And the tendency of humans to crowd together indoors during the winter months made the virus’s journey from person to person a short one.

Turn-of-the-century medical experts, well versed in the germ theory, assumed that some life form, invisible to the naked eye, caused smallpox. But they could only guess at its nature. “The contagious principle, probably a microbe, has not been discovered,” declared an authoritative 1899 pamphlet, prepared by Marine-Hospital Service scientists for Surgeon General Wyman. Since the introduction of the germ theory, European and American scientists had hunted for the disease agent under their microscopes. A few reported seeing traces of smallpox “germs.” Orthopoxviruses are among the largest known viruses, but they are still extremely small. According to one modern writer, it would take three million of them, laid out in rows, to pave over a standard typographic period. An actual sighting would not be possible until the invention of the electron microscope in the 1930s. In 1947 Canadian and American scientists finally viewed the particles, or virions, of variola.27

Since that time, variola virions have often been called bricks, because of their shape: a three-dimensional rectangle with slightly rounded edges. The name fits for other reasons as well. Each virion is made up of a combination of a hundred different proteins, which interlock in a structure so durable that it enables the virions to survive for a time in the open air. The knobby protein exterior of each brick protects the genetic jewel within: a molecule of double-stranded DNA. By attaching itself to and then penetrating a susceptible cell, usually in the mucous membranes of the throat or lungs, a single virion has the power to trigger an unstoppable process of genetic replication that can turn a healthy person into a corpse.28

For all of its mysteries, the clinical features of smallpox were fairly well understood in January 1899, when Surgeon General Wyman issued his “Précis upon the Diagnosis and Treatment of Smallpox.