What Ms. Burkett and Ms. Miller experienced wasn’t the norm. Many of the conditions that can raise a person’s risk for severe Covid — excess weight, heart disease, diabetes, smoking — are highly influenced by environment and behavior, not just genetics. A person’s history of fighting off other coronaviruses, like those that lead to common colds, might also affect their likelihood of developing a serious case of Covid.

Some researchers have also floated the idea that the amount of coronavirus a person takes in may have an impact on the severity of disease, a trend that has been documented with other infections.

“It’s the difference between having your immune system being actually able to squash the infection, or having a much harder time fighting it if all your cells become infected at the same time,” said Juliet Morrison, a virologist at the University of California, Riverside.

Michael Russell, 29, says he wonders if he sniffed up more of the virus than his twin brother, Steven, did this summer, in the days after they gathered with their family for the Fourth of July.

Both brothers began experiencing symptoms shortly after the celebrations ended, around the time Steven headed back to his home in Arlington, Va. The virus saddled Steven with a scratchy throat and a headache — a “light, cold-like” illness, he said.

A few days later, Michael, who was living at home with his parents, came down with much more severe symptoms: a sore throat, chills, shortness of breath and fatigue that relegated him to his bed for an entire day. About two weeks passed before he could smell or taste the cinnamon-dusted popcorn he regularly snacks on.

The twins’ parents came down with bad Covid symptoms as well, so Michael had to isolate with two other infected adults. Hunkering together in the same house may have exposed him to a larger dose of the virus, the brothers said. But, they added, that’s just a guess.



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