According to the World Health Organization (WHO), patients with Covid-19 typically recover from the disease in about two weeks' time, but there are a number of patients who have had Covid-19 for months—and experts aren't entirely sure why they can't shake the disease.

What we know (so far) about the long-term health effects of Covid-19

Meet the Covid-19 'long-haulers'

The Atlantic reports that some patients in online Covid-19 support groups are reporting that they've been experiencing symptoms of the disease for one to three months—and some have even begun calling themselves Covid-19 "long-haulers" or "long-termers."

Cara Schiavo, a 31-year-old woman living in New Jersey, had a fever, chest pain, and shortness of breath, and she tested positive for the new coronavirus on March 10. Four weeks after testing positive for the virus, her symptoms began to subside. "I felt like I was getting back to my old self," she said. "I started walking, exercising, and even told family and friends [I'd] recovered."

But her symptoms returned one week later, and she also began experiencing dermatological and gastrointestinal symptoms associated with Covid-19. Schiavo said she sought care for the returning and new symptoms, but her doctor told her they likely were due to anxiety.

Angela Aston, a 49-year-old RN in Texas who contracted the new coronavirus in late March while treating a patient with Covid-19, had returned to work on April 23 after she was free of a fever for 72 hours. However, toward the end of her shift, she felt "shaky and weak," Aston said. The next day, Aston again had a fever and shortness of breath. "I was confused [and] anxious," she said.

Aston's been retested for the new coronavirus three times, and received one negative result—followed by two positive results. She said she hasn't returned to work since she began feeling symptoms again in late April because her doctors are unsure of whether she's still infectious.

"The [CDC's] return-to-work guidelines say three days no fever, but those guidelines are not appropriate for me," Aston said. "People freak out if a person with recent Covid-19 has an elevated temperature and wants to be around them. Even if it has been 10 days with no fever."

Vonny LeClerc, a 32-year-old journalist in Scotland, started showing symptoms of Covid-19 on March 16. Within days, she developed a cough, chest and joint pain, and a prickling sensation on her skin. LeCLerc's condition began to improve after she rested for a week.

But on day 12, LeClerc's symptoms returned—along with a handful of new ones, including an intermittent fever and loss of taste and smell. And at day 66 since LeClerc first developed symptoms of Covid-19, she was still experiencing them.

"Before this, I was a fit, healthy 32-year-old," she said. "Now I've been reduced to not being able to stand up in the shower without feeling fatigued. I've tried going to the supermarket and I'm in bed for days afterwards. It's like nothing I've ever experienced before."

LeClerc has been unable to get a test for the new coronavirus, but she said "every doctor [she's] spoken to says there's no shadow of a doubt that this has been Covid."

Why the symptoms won't go away

Experts aren't entirely sure why some patients are experiencing symptoms of Covid-19 for so long, though there are a few theories that might explain the matter.

Some experts have suggested that the new coronavirus could be reactivating in the patients' bodies—a phenomenon that occurs with some other viral diseases, such as herpes. Bernard Chang, an emergency physician and psychologist at Columbia University, explained that viral reactivation "is the concept that a latent or 'not active' virus that is already within your system 'awakens' or switches to an active phase and begins causing symptoms in patients."

However, whether the new coronavirus is able to reactivate has yet to be proven or disproven.

Daniel Kuritzkes, chief of the division of infectious diseases at Brigham and Women's Hospital, said he doesn't think the new coronavirus can reactivate, because it spreads differently than known reactivating viruses.

Others have raised the possibility that patients are becoming reinfected with the new coronavirus. According to WHO, it is still unclear whether individuals infected with the new coronavirus develop any immunity against the virus—though some research has suggested immunity might occur.

Some experts have suggested that relapsing symptoms simply may be a characteristic of Covid-19. Jeremy Faust, an ED doctor at Brigham and Women's, cited the common cold as an example. Faust explained that a common cold can present with mild viral symptoms that can "return or get a little worse after initially recovering."

Along those same lines, some Covid-19 patients "may simply have coronavirus symptoms that come and go, but slowly improve over time, like a pendulum running out of energy," Faust said. "I can't think of any illness that doesn't ebb and flow in terms of symptom resolution, to some extent. So in that regard, coronavirus may be no different."

Another possibility is that the persisting and recurring symptoms are caused by a separate, chronic post-viral syndrome, experts, including Kuritzkes, have said. But Kuritzkes said he doesn't believe such syndromes are common.

While experts don't yet have many answers for Covid-19 long haulers, Kuritzkes said such patients shouldn't "get discouraged." He said, "Based on what we know about coronaviruses generally, it is very likely that all of these individuals will completely recover" (Yong, The Atlantic, 6/4; Lowenstein, Vox, 6/4).