The fight against coronavirus is global. But the guidelines on whether you should wear a face mask as part of that fight are often completely different from place to place. That means that, for a lot of people, whether you wear a face mask when you leave the house is basically up to you. Here’s where almost every expert agrees: If you have Covid-19, and you leave the house, you should wear a mask. Masks help keep sick people from spreading their germs. Most of the uncertainty around mask use is related to a totally separate question: Whether healthy people should also wear masks to protect themselves from infection. The truth is that no mask can actually guarantee that you won’t get sick; experts say one of the most dangerous assumptions about face masks is that they basically make you invincible. Masks have to be used correctly to offer any protection at all, and they’re most effective if used alongside other preventative measures like hand-washing and social distancing. But experts also say that the question of whether healthy people should wear masks matters a lot less when you consider one of Covid-19’s most dangerous characteristics: Because of the disease’s long incubation period, and the high occurrence of infected people who never feel symptoms at all, it’s almost impossible to be completely sure that you don’t already have it. And that means the safest course of action is ultimately for everyone to behave like a sick person; in other words, to wear a mask.