evidence: No single study is perfect or definitive; in fact, such studies would be impossible to perform. But there is a lot going for mask-wearing, and little or no evidence that wearing a mask causes harm. [...] - Do masks reduce the severity of infection? Because masks can reduce the "dose" of virus exposure, and because a lower amount of exposure might cause less severe infection, some have suggested that universal mask wearing might induce immunity with less lethal infections. This remains controversial and unproven, however, and should not be assumed to be true. [...] The best available evidence shows that mask-wearing can help reduce spread of COVID-19 infections, keep waves of serious illness from overburdening our hospitals, and save lives — possibly even yours.