It’s useful to study these problems — to identify their mechanics and understand them programmatically, making each one comprehensible enough to fix. Our faith in finding solutions is strong enough, in fact, that we sometimes forget “epidemic” is only a metaphor for something much more resistant to treatment.

It was around 400 B.C. that Hippocrates wrote “Of the Epidemics.” In his Greek, the word “epidemic” combined “upon” (epi) with “the people” (demos) — and “epidemiology” meant the literal description of what was occurring “upon the people” right in front of him, right down to earthly afflictions like dysentery. (“During summer and autumn,” he wrote, “there were dysenteric affections, attacks of tenesmus and lientery, bilious diarrhea, with thin, copious, undigested and acrid dejections, and sometimes with watery stools.”) This is the cool rationalism that remains the gold standard for epidemiologists today: Observe what afflicts people, identify its routes of transmission, then mobilize a response to eradicate it.

This is one of the jobs of the Centers for Disease Control and Prevention, which has defined an epidemic as “an increase, often sudden, in the number of cases of a disease above what is normally expected in that population in that area.” It also studies public-health issues like suicide and drug overdoses (though not gun violence; in 1996, Congress blocked it from any research that might advocate gun control). Each of these tragedies can seem hyperpersonal, each case presenting a unique set of emotional circumstances; in the case of suicide especially, the totality of a person’s life may be put under a microscope in an effort to understand what happened. But when epidemiologists look at tens of thousands of suicides or overdoses at once, they can begin to sketch out patterns — and to locate intervention points that would have been invisible at an individual level.

What’s striking, lately, is the way that logic has spread further beyond the realms of the medical, into spaces once considered too messy and human for science to fully apply — into things like politics and media and popular culture. That was certainly the impulse around the turn of the millennium, when people were latching onto the biologist Richard Dawkins’s idea of the “meme” to explain how behaviors and ideas replicated themselves, passing back and forth among individuals like genes or viruses. (Things that replicated themselves this way on the growing internet were, of course, “viral” as well.) And it was in 2000 that the current epidemic of epidemics found a Patient Zero. One of the year’s best-selling books was Malcolm Gladwell’s “The Tipping Point,” which popularized the application of epidemiology to social phenomena ranging from the success of “Blue’s Clues” to suicide among teenagers in Micronesia. The best way to understand these or “any number of the other mysterious changes that mark everyday life,” Gladwell wrote, “is to think of them as epidemics” — right down to a mid-’90s “epidemic” of people wearing brushed-suede Hush Puppies.