The threat of coronavirus transmission by asymptomatic carriers

Asymtomatic coronavirus spreaders dont show visible symptoms, like coughing or fever. But they could well be passing on the coronavirus. Experts argue many patients appear most contagious right before the onset of symptoms. Models suggest half all transmissions can be traced back to people, before they get sick - if they get sick at all. Many doctors say therefore only mass testing can break the chain of transmission. 

But the US Centers for Disease Control and Prevention has changed its testing guidelines to exclude people who don't have symptoms, even if they've been recently exposed to the virus. 

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