It was the perfect storm of disease transmission: a tourist destination packed with foreign visitors and unvaccinated local residents during a busy holiday season. I'm not talking about last month at Disneyland—the epicenter of a measles outbreak that's infected more than 50 people in four states and two countries to date—but the Château de Versailles in April 1774, where smallpox struck more than 50 people, including King Louis XV himself.
In 18th-century Europe, smallpox was a scourge feared by kings and commoners alike. It was highly contagious, grossly disfiguring, and often fatal. But it was also preventable. Smallpox inoculation—which by then was the norm across Asia and the Middle East—was introduced in the West by Lady Mary Wortley Montagu. Herself a smallpox survivor, the English ambassadress to Turkey had witnessed the practice in Constantinople, and upon her return to England in 1718 she became its biggest advocate. Initially, London society found the practice shocking, but by the end of the century, inoculation had been adopted throughout most of Europe.
France was one of the last holdouts. Though inoculation was common in northern Europe by the early 1770s, it was still regarded with suspicion in France, and with good reason: Improperly performed, it could result in infection and even death. Before Edward Jenner developed the first vaccine to combat smallpox in 1798, the practice of inoculation involved injecting a small amount of pus from the lesions of a smallpox sufferer under the skin of a healthy patient—just enough to stimulate the production of antibodies without triggering a full-blown case. (Jenner’s vaccine used cowpox, a mild virus that conferred immunity to its deadly relative; the term "vaccine" is derived from the Latin vacca, meaning cow.) While the likes of Voltaire and Diderot championed inoculation, the conservative French medical establishment resisted the new and risky practice. After a 1762 epidemic was blamed on inoculation gone wrong, it was even banned in Paris for five years.