Von Miller of the Denver Broncos called the disease a ?surreal? experience and said he struggled at first to work out. Other athletes have endured lingering lung and other health issues.

It was the end of March, and Josh Fiske, a urologist from Livingston, N.J., was in the hospital fighting anJoshua Fiske has attacked his recovery from Covid-19 like a punishing, long training run. uphill battle against the coronavirus. Just a week earlier, he had easily jogged a five-mile route around his neighborhood. But his body was failing him now.

His oxygen levels dipped dangerously low, and his fever rocketed to a worrying 104 degrees. Shifting his body in bed exhausted him. Walking a few steps felt like ?hiking in thin air.? Opening a bottle of iced tea was ?a huge task.?

Fiske kept fighting, though, and eventually, with the help of his doctors, he turned a corner. Yet even as he did, even as he seemed assured of avoiding the worst outcomes of the virus, a different sort of anxiety consumed him.

?I started to think, ?Am I going to be able to run again? Am I going to be able to walk the golf course??? said Fiske, 46, who does a marathon or half-marathon every year. ?These are things I love to do.?

The coronavirus has infected millions of people around the world. Athletes tend to view themselves as perhaps better equipped than the general population to avoid the worst consequences of the disease the virus causes, Covid-19.

Yet interviews with athletes who have contracted the virus ? from professionals to college athletes to weekend hobbyists ? revealed their surprise at the potency of its symptoms, struggles to reestablish workout regimens, lingering battles with lung issues and muscle weakness, and unsettling bouts of anxiety about whether they would be able to match their physical peaks.

And with sports leagues around the world scrambling to restart play, more athletes could soon be taking on a significant amount of risk.

?It definitely shook me up a bit ? it was very surreal, you know?? Von Miller, a linebacker for the Denver Broncos who contracted the virus, said in an interview. ?My biggest takeaway from this experience is that no matter how great of shape you are in physically, no matter what your age is, that you?re not immune from things like this.?

Miller, who has had asthma his whole life, said he was left shaken up by shortness of breath and coughing when he tried to sleep. He said he felt himself ?fatiguing faster? when he first tried working out again in his home gym, but that now he was training ?full-on? again.

Experts warn that the virus does not discriminate.

That was the lesson Andrew Boselli, an offensive lineman at Florida State, learned as members of hisFlorida State lineman Andrew Boselli, center, said the virus made him “the sickest I’ve ever felt.” family ? including his father, Tony, 47, a former N.F.L. lineman ? began showing symptoms in March.

?I knew I was young and healthy,? said Boselli, 22, who moved home to Jacksonville, Fla., after the university closed its doors. ?I play Division 1 football, and I?ve been training my butt off all winter and spring. I thought I had no worries. I wasn?t going to get it.?

That bullish attitude faded days later, when he awoke feeling sluggish and short of breath. That night, his body temperature climbed to 104.

?It was the sickest I?ve ever felt,? said Boselli, who continued to feel shortness of breath and fatigue for about week and a half.

In Italy, Paulo Dybala, an Argentine player with Juventus, described his own unnerving experience dealing with respiratory symptoms.Paulo Dybala, center, of Juventus, described severe respiratory problems.

?I would try to train and was short of breath after five or 10 minutes,? Dybala said in an interview with the Argentine Football Association, ?and we realized something was not right.?

Panagis Galiatsatos, a pulmonary physician and assistant professor at Johns Hopkins, said that, like much about the disease, the long-term consequences for athletes who contract it are not fully understood. Athletes, though, represent interesting case studies for doctors, given their generally good baseline health and nuanced awareness of their own bodies.

?Patients who are athletes, I love them, because they will pick up subtle changes sometimes way before even the tests identify a disease,? Galiatsatos said.

Galiatsatos singled out three complications from Covid-19 that could be of particular concern to athletes.

First, coronavirus patients, like anyone with a serious respiratory infection, were at risk for long-term lung issues. He often saw patients ?who three months ago had a bad virus and still can?t get their breathing back to normal.?

?Sometimes a bad virus creates an airway disease similar to an asthma,? he said. ?They can ravage the lungs, where the lungs were rebuilt, but not well, and patients are stuck with an asthmalike reactive airway disease situation.?

Another complication that Galiatsatos considered particularly concerning to athletes, and one that experts were still trying to wrap their heads around, was the high incidence of blood clots that doctors were seeing in coronavirus patients. People diagnosed with blood clots, and prescribed blood thinners, are typically discouraged from participating in contact sports.

Finally, Galiatsatos said people unfortunate enough to be placed in intensive care could deal with ?I.C.U. acquired weakness.? Patients placed on ventilators and confined to a bed often lost between 2 and 10 percent of their muscle mass per day, he said.

Ben O?Donnell, a triathlete who lives in Anoka County, Minn., lost 45 pounds during a four-week hospital stay during which he was placed on a ventilator and a short-term life support machine.O?Donnell was told he may never regain his full lung capacity.

O?Donnell, 38, a former college football player who completed an Ironman race a couple of years ago and was planning on doing another this fall, said he was pulled back from the brink of death after struggling with dangerously low levels of oxygen and kidney and liver failure in the intensive care unit.

In mid-February, in anticipation of ramping up his training, O?Donnell had completed a two-day, comprehensive physical exam and received a clean bill of health. Doctors believe he contracted the virus five days later.

Back at home after his harrowing month in the hospital, O?Donnell has set his sights on competing in an Ironman race in Arizona this fall. He acknowledged it was a lofty goal.

?They?re not sure if I?ll ever get full lung capacity back,? he said. ?I may or may not.?

Had he not contracted the virus, O?Donnell, an executive at a chemical company, would be doing three runs, three swims and three bicycle workouts per week at this point in his training cycle. But the virus derailed his life plans.

After returning home, he needed a walker just to go out to the mailbox at the end of the driveway. In his first attempt to exercise, two days after he left the hospital, he walked for seven minutes at a speed of 1.2 miles per hour using supplemental oxygen. He has been trying to add a minute of time, and a bit of speed, each day.

O?Donnell said he was struggling with ?a fair amount of doubt? about his ability to get back in shape for the race. But he has motivated himself with the secondary goal of raising money for coronavirus relief, and he has been repeating the same mantra ever since he was struggling in his hospital bed: ?Don?t stop. Don?t quit. Keep moving forward.?

This mentality has helped other athletes who have been hit with serious symptoms.

The 29 days Tsang Yee-ting spent in the hospital were the most she had been away from a karate mat since being introduced to the sport at age 6. A member of the Hong Kong national team, Tsang, 27, contracted the coronavirus in March while preparing to qualify for the Summer Olympics.

Tsang Yee-ting, left, a karateka from Hong Kong, worried about regaining her form. She says she has.

For the next month, she battled a range of symptoms, the worst a searing pain that engulfed the lower half of her body. Walking was a struggle. Lying down offered no relief. As she fought a virus that doctors were still learning about, ?all sorts of thoughts? about her body and about her future spiraled through her mind, she said.

?Of course I was worried,? Tsang said. ?Karate is my life.?

But even as the virus and isolation from her family levied an ?emotional toll? on her, Tsang resolved to stay as active as possible to keep herself sane. She acquired elastic bands and, on days when her body felt strong enough, completed mini-workouts in the tight confines of her hospital room.

?Battling the virus was like training for a competition,? said Tsang, who said she now felt normal again and has been training with her teammates over video.

Fiske, the urologist, has been working, very slowly, to get back into shape after his weeklong I.C.U. stay in March.

Fiske said he found himself relying on the same mental calisthenics he might have used to get through a punishing long-distance run ? ?when you?re having a tough time, and you decide to do another mile or two to see if you can break through it? ? to stave off negative thoughts about his recovery.

Since returning home, his focus has been on regaining lung strength. He could not walk up the stairs without becoming ?totally gassed.? He could not hold a conversation.

But it pained him to think that he might not be able to run, to golf or to exercise with his two teenage sons. So he has persisted with breathing exercises and laboring jogs through his neighborhood. Recently, he was able to finish his old five-mile route ? albeit at a different pace.

?They?re slow,? he said of the miles. ?But they?re there.?


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