Yet there do seem to be situations in which a few individuals spark large outbreaks. With Covid-19, it is not yet known whether those highly infectious people include individuals with silent infections who do not realize they are sick, said Dr. Thomas Frieden, former director of the Centers for Disease Control and Prevention and chief executive at Resolve to Save Lives, an initiative of Vital Strategies. More likely, he adds, superspreading events may involve people with symptoms that linger but who are not sick enough to stay home.

Or they could involve infected people who shed an unusual amount of virus — a poorly studied factor that might be due to variations in the amount of virus in the aerosol droplets from a patient’s cough or the amount of infectious virus in feces, for example.

No matter what the cause, public health measures, like avoiding crowds, and what Dr. Frieden calls cough hygiene, can prevent a superspreading event, he said.

Medical history is replete with stories of superspreading in outbreaks of parasitic disease, tuberculosis, measles and other illness.

There is Mary Mallon, a cook better known as Typhoid Mary, who spread typhoid fever to more than 50 people in the early years of the twentieth century. She herself was not ill but was asymptomatic — silently infected with typhoid.

Superspreading also played important roles in outbreaks of two other coronaviruses, SARS and MERS.

“The MERS-CoV outbreak in South Korea was driven primarily by three infected individuals, and approximately 75 percent of cases can be traced back to three superspreaders who have each infected a disproportionately high number of contacts,” wrote George F. Gao, an immunologist and virologist at the Chinese Centers for Disease Control and Prevention in Beijing, in a recent paper.