When an infection erupts the way coronavirus has exploded in Wuhan, China, and elsewhere in the world, public-health experts try to gauge the potential for an epidemic—or, worse, a pandemic—by calculating the pathogen’s basic reproduction number.

The figure, generally written as R0 and pronounced “R naught,” is an estimate of how many healthy people one contagious person will infect. Because viruses spread exponentially, a few cases can quickly blow up to an overwhelming number. An R0 of two suggests a single infection will,...