The death of the Broadway actor Nick Cordero from Covid-19 has shaken people far beyond the theater world, in large part because he was just 41 and reportedly had no underlying health conditions.

Medical experts said that Mr. Cordero’s death underscored a multitude of unknowns about the coronavirus — including the ways it could imperil even young, healthy people who did not appear to be at increased risk of contracting severe disease.

“The idea that ‘I’m young, I’ll be fine’ is not an idea that we can completely subscribe to,” said Dr. Utibe Essien, a physician and health equity researcher at the University of Pittsburgh.

Amanda Kloots, Mr. Cordero’s wife, has said that he had no known pre-existing conditions that might have worsened the course of his disease. As more data emerges, serious cases of younger, healthy people like him are becoming less of an anomaly, doctors said.

“A young person who has no real medical comorbidities, but gets super sick and ends up on multiple support machines” is a clinical portrait that doctors are now seeing “a lot,” said Dr. Taison Bell, a physician specializing in infectious disease and pulmonary and critical care at the University of Virginia.

These concerns are growing more pressing as the average age of coronavirus patients trends down in states like Arizona, Florida and Texas. “In general, younger people will do better than older people,” Dr. Bell said. “But once someone becomes critically ill, a lot goes out the window.”

Mr. Cordero fell into an age bracket with a Covid-19 death rate estimated at 0.4 percent of confirmed cases, compared with nearly 15 percent for those over 80.

But for people of any age, diabetes, immune disorders, obesity and chronic diseases that affect the heart, lungs and kidneys are among the conditions thought to raise the risk of severe disease and death.

Though he was said to have had none of those issues, the course of Mr. Cordero’s illness was peppered with setbacks and complications. After being admitted to a hospital in Los Angeles on March 30, he was determined to be in critical condition, was put on a ventilator and underwent treatment with dialysis and a specialized heart-lung bypass machine, called extracorporeal membrane oxygenation, or ECMO. He spent six weeks in a medically induced coma. In April, excessive blood clotting forced doctors to amputate his right leg.

Mr. Cordero died on Sunday, more than three months after he was stricken with the coronavirus.

Doctors have been continually surprised by the progression of Covid-19 in different patients, but they have painted a general trajectory: Those who develop symptoms do so within two weeks of infection; patients who are saddled with severe disease tend to rapidly decline about seven days into their illness. Mild cases resolve in about two weeks time; more serious ones can stretch out to three to six weeks, with death — when it happens — occurring shortly thereafter.

But these numbers are built from averages, medians and estimates. Many people plagued by the coronavirus have debilitating symptoms for months, even after being discharged from the hospital. And health workers and researchers have an incomplete picture of the factors that can lead a patient onto a path toward either a speedy recovery, or fast deterioration and death.

The most severe symptoms of Covid-19 are thought to be the result of an overactive immune response, kick-started to wage war with the virus, that ultimately overwhelms the body. Damage wrought by this friendly fire can sometimes incite dangerous bouts of blood clotting — a sequence of events that was especially poorly understood at the start of the outbreak, said Dr. Phyllis Tien, a physician and infectious disease expert at the University of California, San Francisco.

“We know so much more today than we did three months ago,” Dr. Tien said. “But every day, something new happens.”

The announcement of Mr. Cordero’s death arrived just a day after President Trump falsely claimed that 99 percent of coronavirus cases are “totally harmless.” Dr. Bell called those comments “an insult to the people who have died, and contributed to our knowledge of this.” Even for those who have survived, their suffering can be immense and prolonged.

Researchers do not yet fully understand who is at greatest risk of serious disease. Advanced age and a shortlist of medical conditions do seem to play a role, Dr. Essien, the health equity researcher, said. But social factors play enormous roles as well — and those who are marginalized by race, ethnicity, socioeconomic status and more are known to be especially vulnerable to the virus. Latino and African-American residents of the United States are three times as likely to become infected by the coronavirus as white Americans, and are nearly twice as likely to die from the virus.

The list of factors that can predispose a person to severe Covid-19 will probably shift in the months and years to come, Dr. Essien said. And even those who continue to be in a low-risk category should not assume they are in a no-risk category.

Doctors and nurses are still learning, too, Dr. Tien said. But what’s clear so far is this: “Young people are definitely not immune to this disease.”