The masks encourage a warm moist environment surrounding your nose and mouth. If the mask becomes contaminated, it becomes an ideal vector for encouraging infe

The masks encourage a warm moist environment surrounding your nose and mouth. If the mask becomes contaminated, it becomes an ideal vector for encouraging infection -- so if you touch the inside of the mask with your contaminated hands (say you move the mask to touch your face or adjust it or whatever), for example, the mask becomes the opposite of protective. Masks are somewhat effective at reducing spread if the already-infected person is wearing the mask.