Jeff Engman is one of those patients, now called "long haulers."
"I probably was in some of the best shape of my life," said Engman, who added that he likes to work hard and play hard.
But just as he started work on his latest project, the 58-year-old said he was hit hard by COVID-19.
"The fatigue, I was really drained and, you know, could hardly get out of bed," he said.
After a few weeks, Engman thought he had recovered. Then, new problems set in again.
"COVID caused some abscesses in my lungs," Engman said.
And 10 months later, Engman, like many others diagnosed with a coronavirus infection, is still dealing with symptoms.
Dr. Michael Daignault with Providence Saint Joseph explains 'long haulers'
Dr. Michael Daignault, an emergency medicine physician with Providence Saint Joseph Medical Center, says he sees many long haulers.
"Having brain fog, like difficulty concentrating, shortness of breath especially when they're trying to return to their workouts or exercise regimen, and just overall lack of energy, " Dr. Daignault said.
The list of long hauler symptoms can also include body aches, headaches and loss of taste and smell - even if that didn't occur at the height of a person's COVID infection.
The CDC reports that up to 35% of those infected endure symptoms lasting beyond three weeks.
"COVID is a very much a pro-inflammatory state and it causes micro thrombotic or mini-clots to occur at the organ level," Dr. Daignault explained.
Dr. Daignault says research reveals that there are five factors that can raise your risk.
"Being female, being overweight, having a history of asthma, being elderly. And if you had multiple symptoms during your acute phase and infection in your first couple weeks, " he said.
RELATED: This device may be helpful if you're healing from COVID at home
As for Engman, he says he has good days and bad days.
"It's not going away like I thought it would," Engman said. "You kind of wonder if you're, you know, are you ever going to get better?"
Mount Sinai Medical Center in New York has now opened a center for post-COVID patients. It's staffed with doctors who are dedicated to studying and treating long haulers. They're modeling the center after what they did for first responders after 9/11, but on a much larger scale.
CA COVID-19 vaccine tracker: See your status here
Having trouble loading the tracker above? Click here to open it in a new window.