That's why we read so avidly about recurrent outbreaks (such as those in 2012) of Ebola virus disease among villages in Uganda and the Democratic Republic of Congo, and about West Nile fever in the area around Dallas (where more than a dozen have died of it since July). And that's why, early last autumn, heads turned toward Yosemite National Park after the announcement of a third death from hantavirus pulmonary syndrome among recent visitors there.

Humans die in large numbers every day, every hour, from heart failure and automobile crashes and the dreary effects of poverty; but strange new infectious diseases, even when the death tolls are low, call up a more urgent sort of attention. Why?

There's a tangle of reasons, no doubt, but one is obvious: Whenever an outbreak occurs, we all ask ourselves whether it might herald the Next Big One.

What I mean by the Next Big One is a pandemic of some newly emerging or re-emerging infectious disease, a global health catastrophe in which millions die. The influenza epidemic of 1918-19 was a big one, killing about 50 million people worldwide. The Hong Kong flu of 1968-69 was biggish, causing at least a million deaths. AIDS has killed some 30 million victims and counting. Scientists who study this subject--virologists, molecular geneticists, epidemiologists, disease ecologists--stress its complexity but tend to agree on a few points.

Yes, there probably will be a Next Big One, they say. It will most likely be caused by a virus, not by a bacterium or some other kind of bug. More specifically, we should expect an RNA virus (specifically, one that bears its genome as a single molecular strand), as distinct from a DNA virus (carrying its info on the reliable double helix, less prone to mutation, therefore less variable and adaptable). Finally, this RNA virus will almost certainly be zoonotic--a pathogen that emerges from some non-human animal to infect, and spread among, human beings.

ZOONOSES AROUND US