But symptoms like fatigue are often not recognized as being related to myocarditis. And Dr. McManus suspects that the fatigue that sometimes follows a bout with Covid-19 might be caused by this heart problem.

“We think of Covid-19 and influenza as respiratory diseases, and in fact they are,” said Dr. Bruce M. McManus, an emeritus pathology professor at the University of British Columbia. “But the reason many patients reach their demise in many instances is myocardial.”

Some severely ill Covid patients have lung damage. That too can also occur with other viruses, said Dr. Clemente Britto-Leon, a lung researcher at the Yale School of Medicine. He lists some possibilities.

“You can have lung injury and scarring with influenza, with herpes viruses and with cytomegalovirus infections, for example,” Dr. Britto said, referring to a common virus that usually causes no symptoms. All these viruses can wreak damage on rare occasions, he said. “You can have a very severe injury and a lot of tissue destruction.”

Influenza can cause blood clots in the lining of the lungs that look just like the small clots seen in the lungs of some Covid patients, said Marco Goeijenbier of Erasmus University in the Netherlands. It happens when flu viruses infect the lower respiratory tract, an unusual event because most people have some pre-existing protective immunity.

Dr. Goeijenbier wants to study the blood clots that occur in these cases, but previously, with so few patients, he and others resorted to reproducing and studying the effect in laboratory studies and in ferrets — the preferred animals to study flu.

“It was hard to get funding, he said. “Big journals or funders didn’t think it was interesting enough,” he explained.

Covid is changing that.

There is now “a huge cohort of people to study,” said Pamela Dalton, a smell researcher at Monell. But “the big question is, even if you learn everything about SARS-CoV-2” — the formal name of the coronavirus — “how generalizable is it?”