Humankind has battled viruses virtually from the beginning. Take just one example: Variola major, the smallpox virus. Its telltale pocks mark the mummified body of Pharaoh Ramses V, dead now over 3,000 years. From ancient Egypt the disease clawed its way into China, Japan, and India. Over the centuries the human population grew and knotted together in cities, where outbreaks became more severe. Meanwhile, explorers, especially among Europe’s emerging colonial powers, carried the virus to distant lands. By the mid-18th century it was global pandemic, sparing only a few island havens, such as Australia. It killed a third of its victims, including nearly 400,000 Europeans a year. Those spared from death were often left blind or disfigured.

The disease proved implacable, ravaging without regard for class or standing. The only viable treatment was a risky form of inoculation — often the inhalation of dried, infected skin — which produced a weak case of smallpox that doctor and patient alike hoped would prove survivable. Only in 1796 did a rural English doctor named Edward Jenner confirm that cowpox, a related but weaker virus, could serve the same purpose, without the unfortunate side effect of possible death. The method became known as vaccination and marked the beginning of the end for smallpox. In 1979 the World Health Organization declared it eradicated; smallpox exists, officially, only in two high-security research facilities — the Centers for Disease Control and Prevention in Atlanta, Georgia, and the VECTOR Institute in Koltsovo, Russia.