The problem is in most cases, I see people wearing the same mask all day long. From my limited knowledge (excuse me if I'm wrong, maybe a health expert can comment on this), you should change mask every 3 or 4 hours when wearing one. The masks, especially cloth masks, (if they work in the first place) lose effectiveness when they are wet from the water vapor coming out of your breath and wet masks become breeding grounds for all kinds of nasty things. I assume that the bacterial pneumonia results from breathing with wet masks. At least that's what I remember from what the health officials in my country told the public, which seems to contradict what most sources in the US say.