The 33-year-old southpaw says he contracted the coronavirus back in January ... and went into detail with reporters about his experience on Wednesday.

"I was surprised," Britton said. "I think I got it at the hospital when we were having our kid, so it was even worse."

"I was not expecting to be hit that hard by COVID. It wiped me out pretty good."

"The weight loss was surprising and just the after-effects I've been dealing with showed me how serious this can be, even for somebody that's healthy and how that can impact you months even after you've gotten over the roughest symptoms."

FYI -- Britton is 6'1" and reportedly usually weighs around 200 pounds ... in other words, he's in pretty good shape.

"It just wiped me out pretty good for about 10 days," Britton said. "Lost about 18 pounds, which could be good, I guess? But, I wasn't looking to lose 18 pounds and it happened quickly, so that's not good on your body and puts pressure in a lot of places."

Britton says the virus prevented him from throwing for about 3 to 4 weeks during the offseason ... but he's finally starting to feel like himself again.

But, COVID isn't the only health issue Britton's dealing with -- the 2-time All-Star is set to undergo surgery to remove a bone chip in his left elbow ... and is expected to miss several months of the season.