When it comes to spreading viral and bacterial infections, some people are more contagious than others—much more contagious.

Known as superspreaders, they amount to roughly 20% of the population, but they account for transmission of about 80% of certain infectious diseases, scientists estimate. The phenomenon has been observed, among other contagions, during the global SARS outbreak in 2002 and 2003 and as far back as Typhoid Mary, a cook in New York who infected dozens of people with typhoid fever in the early 1900s without...