“There’s this small percentage of people who appear to infect a lot of people,” said Dr. Joshua Schiffer, a physician and mathematical modeling expert who studies infectious diseases at the Fred Hutchinson Cancer Research Center in Seattle. Estimates vary from population to population, but they consistently show a striking skew: Between 10 and 20 percent of coronavirus cases may seed 80 percent of new infections. Other respiratory diseases, like the flu, are far more egalitarian in their spread.

Figuring out what drives coronavirus superspreading events could be key to stopping them, and expediting an end to the pandemic. “That’s the million dollar question,” said Ayesha Mahmud, who studies infectious disease dynamics at the University of California, Berkeley.

In a paper posted Friday to the website medRxiv that has not yet been through peer review, Dr. Schiffer and his colleagues reported that coronavirus superspreading events were most likely to happen at the intersection where bad timing and poor placement collide: a person who has reached the point in their infection when they are shedding large amounts of virus, and are doing so in a setting where there are plenty of other people around to catch it.