By this point, many people can identify symptoms of COVID-19 -- fever, fatigue, shortness of breath -- but fewer know that those can occur after a successful recovery and closely resemble another illness: chronic fatigue syndrome.
"There may well be a post-viral syndrome associated with COVID-19," Dr. Anthony Fauci, the nation's top infectious disease doctor, said at a July news conference organized by the International AIDS Society. "I know, because I follow on the phone a lot of people who call me up and talk about their course."
Although chronic fatigue syndrome is still poorly understood, the Centers for Disease Control and Prevention said as many as 2.5 million Americans may suffer from it.
"About 75 to 80% of chronic fatigue syndrome cases are post-viral in nature," estimated Dr. Mark VanNess, department chair of Health and Exercise Science at the University of the Pacific. "The viral infection and following immune response are precipitating causes for long-term symptoms."
Prior to the COVID-19 pandemic, scientists and doctors already were researching the syndrome's association with Epstein-Barr Virus.
For many people, chronic fatigue syndrome develops after a viral infection, and now it seems some post-COVID patients develop symptoms that resemble those seen among chronic fatigue syndrome patients. This observation has led some doctors to explore whether the two conditions might be connected.
Chronic fatigue syndrome tends to be long-lasting and cause functional impairments -- extreme fatigue following physical or cognitive activity, known as post-exertional malaise,can last for hours, days or even weeks. This prolonged fatigue after exertion makes chronic fatigue syndrome unique, said Dr. Lily Chu, vice president of the International Association for Chronic Fatigue Syndrome.
"For most diseases like heart or lung disease, or even arthritis, post-exertional symptoms happen immediately after the activity," Chu said. "The combination, duration and severity of these symptoms makes life unpredictable for patients."
Scientists' and doctors' understandings of chronic fatigue syndrome have dramatically evolved in recent years. Previously, patients were encouraged to increase their physical activity over time, but now they're encouraged to understand their limitations and rest when that limit has been reached.
"Eliminate physical stress, learn pacing and energy-management methods, educate family members about the illness and intervene in a systematic way," VanNess explained.
Patients are now advised to avoid the pressure to return to work or resume normal activities until symptoms are under control. Patients may also consider wearing a device to monitor their total daily steps so that they can better understand their threshold of activity that leads to symptoms. Patients also should get sufficient rest to help avoid possible flare-ups.
"Most patients are significantly disabled and have had to cut out, reduce or change work, school, family caregiving, social and other activities," Chu explained. "About 25% of patients are so sick they must remain at home or in bed all the time. On average, only 5% of patients return to their pre-illness health, but some degree of improvement may occur in 40%."
Exactly how much or how quickly COVID-19 patients can recover isn't entirely clear -- a CDC report in July said that as many as 25% of those infected didn't feel like their normal selves two or three weeks after testing positive.
It's not yet clear yet why some patients have lingering fatigue after initially recovering from COVID-19, and doctors don't yet know whether a significant portion of those individuals will develop chronic fatigue syndrome. If an individual who recovered from the novel coronavirus is still experiencing fatigue six months later, they should see their doctor.
Dr. Christie Richardson, a second-year psychiatry resident, and Dr. Danielle Weitzer, a fourth-year psychiatry resident, are members of the ABC News Medical Unit.