In the fall of 2009, one of us, Beth, was hit by an illness she suspects was H1N1 flu, which was circulating then. In 2012, the other, Brian, developed a sudden fever, which his doctors said was also likely of viral origin.

Neither of us recovered, and we’re both disabled to this day.

The long-term illnesses that can follow viral infections can be devastating – and are devastatingly common. In 2015, the nation’s top medical advisory body, the Institute of Medicine, estimated that between 800,000 and 2.5 million U.S. residents live with the illness or illnesses awkwardly named myalgic encephalomyelitis/chronic fatigue syndrome (ME/CFS). An estimated three-quarters of these cases were triggered by viral or bacterial infections.

Now, as a new pandemic virus is burning through the world and causing many deaths, researchers are raising alarms that the novel coronavirus and the COVID-19 disease it causes will also leave in its wake a potentially large population with post-viral problems that could be lifelong and, in some cases, disabling.

At the National Institutes of Health and elsewhere, scientists who have been studying post-viral ME/CFS are seizing the opportunity to focus on COVID-19 patients. They want to understand what biological factors separate those who regain their health from those who remain sick.

“We want to look at who recovers and who doesn’t,” said Avindra Nath, the head of clinical neurology at NIH’s Clinical Center in Bethesda, Maryland, who is gearing up to study COVID-19 patients. “It’s quite possible some will never get their health back.”

In addition to emerging reports of damage to lungs, kidneys and hearts, COVID-19 patients are complaining of ongoing crushing fatigue, muscle pain, cognitive problems and other symptoms that anyone with ME/CFS is very familiar with.

“I’m 69 days after my first symptoms and still feeling fatigued,” wrote Canadian comedian Wayne Jones on Twitter, saying he has tested positive for the new coronavirus. “I also have sore eyes, and weakness and headaches that come and go. All signs point to post-viral fatigue.”

Sometimes a precursor to ME/CFS post-viral fatigue syndrome can occur after virtually any viral infection. Symptoms wax and wane; a person can feel fine one day and terrible the next. Exercise or other usual activities can bring fever and often symptoms rushing back. Although there is no known treatment, these problems often resolve on their own.

But patients who have experienced post-viral fatigue syndrome advise the newly ill to rest, rest and rest some more, as returning to normal activities can trigger relapses. If symptoms continue for six months or longer, post-viral fatigue syndrome can convert to a diagnosis of ME/CFS, which is usually lifelong and often devastating. Up to 25 percent of ME/CFS patients are housebound or bedbound for years. There are no treatments approved by the Food and Drug Administration.

A survey organized by a group of COVID-19 patients called the Body Politic shows that many coronavirus symptoms overlap with those of post-viral fatigue syndrome. Fatigue, including severe fatigue, brain fog – problems with concentration and thinking – chills and sweats, and sleep problems were commonly reported among 640 respondents, who were recruited from online support groups. The Body Politic, which includes some patients with expertise in survey design and statistics, calculated that respondents had just a 20 percent chance of being symptom-free 50 days after the illness began. The group cautioned that while the survey suggests many respondents had long illnesses, the results may not apply to the broader population of COVID-19 patients.

The novel coronavirus is too new to say whether such lingering symptoms represent an extended COVID-19 illness or a post-viral syndrome. But researchers suspect at least some COVID-19 patients will remain sick and eventually develop post-viral ME/CFS. Their suspicions are grounded in previous research. Studies on other outbreaks – including those of Epstein-Barr virus, Ross River fever and Q fever – have shown that up to 12 percent of people who fell acutely ill never regained full health. Many were eventually diagnosed with ME/CFS.

After the first SARS epidemic, which sickened 8,000 people worldwide in 2002-2003, one study found that 27 percent of 369 survivors of that variety of coronavirus met the criteria for chronic fatigue syndrome several years later. If figures like this are seen with this novel coronavirus, SARS-CoV-2, which epidemiologists warn will eventually infect 50 to 70 percent of all U.S. residents, our nation is on track for a devastating wave of millions of patients with chronic post-viral illnesses.

“The most important study of several we’re starting is to get coronavirus-infected patients and follow them over time to see if they convert to ME/CFS,” said Ron Davis, a geneticist at Stanford University. His adult son, Whitney Dafoe, is seriously ill with the disease.

“If you point out the fact that you might not ever get over COVID-19, it’ll put people’s attention on this problem,” Davis said. “We need doctors to be reading COVID-19 information and be on the lookout for people who never get well. I think that’s extremely important.”

Mady Hornig, an immunologist at Columbia University’s Center for Infection and Immunity, said her group plans to follow COVID-19 patients with neurological symptoms, including loss of smell, to see who develops ME/CFS. She plans to recruit African American, Hispanic and American Indian patients, who are underrepresented in ME/CFS research. Hornig herself has had COVID-19 symptoms for six weeks, including a racing heart beat, which can signal issues with the autonomic nervous system.

At Stanford, Davis is partnering with the nonprofit Open Medicine Foundation. Already, clinics funded by the OMF are collecting blood from COVID-19 patients and asking them to wear activity tracking devices. “We want to study them at three months, six months, longer, and see who will be diagnosed with ME/CFS,” Davis said.

Hornig and Davis both say that watching COVID-19 patients could speed the search for drugs to reverse whatever goes awry in post-viral illnesses.

“Scientifically I think the most important thing to look at is . . . why they convert to ME/CFS. What’s different about them?” Davis says. “That could be extremely valuable, that could give us a valuable clue to … how to treat it.”