Davis said Valley Fever is the "working diagnosis" -- tests have come back negative -- of the fungal disease doctors suspect he has but he said he's experienced no symptoms, the New York Daily News reported.
The infection occurs when spores of a fungus enter the body through the lungs but experts say most people never have symptoms. Valley Fever can cause cold- or flu-like symptoms or the same symptoms as pneumonia.
"If I had a cough or felt sick, I'd have maybe worries or something like that, but I feel great," Davis said. "It's kind of weird. I'm not coughing, I'm not throwing up blood, anything. It's not even hard to breathe."
A doctor told him he could play but to avoid getting extremely fatigued, he said.
In 2009, Conor Jackson, a former Diamondbacks outfielder, played only 30 games because of Valley Fever.
Mets General Manager Sandy Alderson said doctors had spotted something on Davis' chest X-ray that led to the "hypothesis" he had Valley Fever.
Davis is to undergo a follow-up test when the team returns to New York, Alderson said.