Staff members (in black) check the body temperature of passengers after a train from Wuhan arrived at Hangzhou Railway Station in Hangzhou, China's eastern Zhejiang province. AFP via Getty Images
Chinese authorities have known about asymptomatic carriers of the CCP virus since early February, but they did not warn the public of the infection risks imposed by this group of people until recently. These asymptomatic people are able to transmit the virus to others. If they never have a diagnostic test, no one will know that they are virus carriers, as they look healthy.
