Carl Zimmer reports:

At a May 30 birthday party in Texas, one man reportedly infected 18 friends and family with the coronavirus.

Reading reports like these, you might think of the virus as a wildfire, instantly setting off epidemics wherever it goes. But other reports tell another story altogether.

In Italy, for example, scientists looked at stored samples of wastewater for the earliest trace of the virus. Last week they reported that the virus was in Turin and Milan as early as Dec. 18. But two months would pass before northern Italy’s hospitals began filling with victims of Covid-19. So those December viruses seem to have petered out.

As strange as it may seem, these reports don’t contradict each other. Most infected people don’t pass on the coronavirus to someone else. But a small number pass it on to many others in so-called superspreading events.

“You can think about throwing a match at kindling,” said Ben Althouse, principal research scientist at the Institute for Disease Modeling in Bellevue, Wash. “You throw one match, it may not light the kindling. You throw another match, it may not light the kindling. But then one match hits in the right spot, and all of a sudden the fire goes up.”

Understanding why some matches start fires while many do not will be crucial to curbing the pandemic, scientists say. “Otherwise, you’re in the position where you’re always one step behind the virus,” said Adam Kucharski, an epidemiologist at the London School of Hygiene and Tropical Medicine. [Continue reading…]