Underwood, 28, said that he started experiencing symptoms a few days earlier.
“I want to let you guys know: I’m 28, I consider myself pretty healthy, I work out regularly, I eat healthy, and I became symptomatic a few days ago, got my test results back today, and they are positive,” he said in an Instagram video.
The reality star said the virus, officially named COVID-19, has been draining.
“It’s been kicking my ass, just to put it pretty bluntly,” he said. “The main thing is I can’t even walk up a flight of stairs without being out of breath or go to the bathroom without having to sit down because I’m exhausted.”
Underwood said that he is isolating himself at the home of his girlfriend Cassie Randolph’s parents, but that the rest of the home may get infected as well.
“I’m doing okay, I’m at Cassie’s family’s house in Huntington [Beach, California], so I’m fortunate to have them. Unfortunately that means we’re all in this together, at this point. So we’ll keep you posted, but we’re in good spirits.”
Underwood said that everyone, even younger adults, need to take COVID-19 seriously.
“So, I guess the reason I’m sharing this is not to cause fear or panic, but to hopefully encourage you guys to stay home, do your part, take care of yourselves, take care of one another,” he said. “… I just want to encourage everybody to stay at home. Stay in your house and do your part and take care of one another.”
Underwood and Randolph had been making a point to practice social distancing over the last week, but on Monday, they decided to go for a swim in the ocean. Randolph was then stung by a jellyfish, and had an allergic reaction to the venom, requiring a trip to the emergency room.
“Mother Nature sent us a warning shot to get back inside,” Underwood joked.
As information about the coronavirus pandemic rapidly changes, PEOPLE is committed to providing the most recent data in our coverage. Some of the information in this story may have changed after publication. For the latest on COVID-19, readers are encouraged to use online resources from CDC, WHO, and local public health departments and visit our coronavirus hub.