Per the Associated Press:

“There’s no evidence of masks leading to fungal or bacterial infections of the upper airway or the lower airway as in pneumonia,” said Davidson Hamer, infectious disease specialist and professor of global health and medicine at Boston University.

Hamer noted that bacterial growth could occur, in theory, if someone wore a mask that was already contaminated with moisture and became moldy. “I don’t know why anybody would do that. Theoretically, it could happen, but it’s highly unlikely with just typical mask use,” he said in a phone call with The Associated Press.

There’s no evidence that wearing face masks cause any harm besides some discomfort, Hamer said. However, he added paper masks that become visibly wet should be discarded. ” (1)

(1) https://apnews.com/article/archive-fact-checking-9072351115