Thomas Perls is a professor of medicine at Boston University. The Centers for Disease Control and Prevention has changed its policy and is now advising everyone, whether or not they have symptoms of COVID-19, to cover their face with a mask or cloth covering whenever social distancing is difficult to maintain. To be clear, the CDC is not saying you should wear a mask wherever you go, but rather in places where people congregate, including grocery stores and public transportation and ride-shares.Where to get a face mask if you’re not a medical workerThe shift in recommendations reflects growing evidence that COVID-19 can be transmitted by a person’s exhalations and normal speech but also the fact that people are not effectively covering their sneezes and coughs. The stealth virusCOVID-19’s middle name should be “stealth.” People can be shedding virus for one to three days before showing any symptoms — including no coughing, sneezing or fever — in what’s called “presymptomatic transmission.” A Singapore study suggests that 10% of infections are attributable to presymptomatic spread. A study of the 3,711 passengers and crew on the Diamond Princess cruise ship indicates that close to 1 in 5 COVID-19 carriers never develop symptoms. Some these

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