Almost all of us have lived almost all our lives in this Golden Age, a very recent phase in the endless struggle between us and our pathogens. Due mainly to clever interventions, but possibly to some biological evolution as well, This Golden Age of Health began to emerge at the end of the 19th century, but it really took hold only after 1945 or so. Recall that as late as 1918-19 a single epidemic of influenza killed 50-100 million people, some 2-5% of the global population, far more than the murderous violence of World War I (1914-18).

This state of affairs makes it easy to forget that what came before was a centuries-long Golden Age of Pathogens—a direct result of globalization. With the emergence of the plague bacillus in the 14th century and the newly interconnected world after the voyages of Columbus, da Gama and others in the 16th century, pathogens had vast new opportunities to prosper. These took the form of frequent epidemics that brought great suffering and grinding wastage of human life, mainly children, via endemic diseases. Smallpox led the way, followed by a half dozen other deadly killers such as influenza, whooping cough (pertussis), measles, and mumps, all of which became more or less global diseases. Some world regions in addition suffered from diseases specific to certain environments, such as yellow fever, which is carried by warmth-loving mosquitoes.

Pathogens are always mutating and evolving, in effect trying to outwit our drugs, just as medical researchers are trying to outwit the pathogens. For two generations now we have had the upper hand. In the Golden Age of Health, the benefits may be far from evenly distributed around the world, but they have reached everywhere. Even the poorest and least healthy countries by 2014 had life expectancies of 46. As recently as 1900, the U.S. population’s life expectancy was under 46.

Could the Golden Age of Health come to an end? Yes, it could. Ebola could surmount our hastily erected defenses and spread far and wide. At the moment the best guesses are that at least 10,000 people in West Africa carry the virus. Some of them are on the move, carrying it to neighboring countries and distant airports. And of course there is more than ebola. Vaccination regimes put in place decades ago were successful enough to drive smallpox to extinction and to make other infections trivial concerns. That very success has tempted people and governments to skip vaccination, allowing resurgent infection among small children. The Golden Age of Health is a wonderful but recent and fragile thing. It is all most of us have ever known, but we must not presume it is here to stay. Ebola may be trying to tell us something.

McNeill is a history professor at Georgetown University and a prominent environmental historian. He is the author of Something New Under the Sun: An Environmental History of the Twentieth-century World.