Most coronavirus infections are spread by people who have no symptoms, the US Centers for Disease Control and Prevention said in newly updated guidance.
It's one of the main reasons mask use is so important, the CDC said.
"Most SARS-CoV-2 infections are spread by people without symptoms," the agency said in a section of its website devoted to explaining the science of how to use masks to control the spread of the virus.
"CDC and others estimate that more than 50% of all infections are transmitted from people who are not exhibiting symptoms," it added in the guidance posted Friday.
"This means at least half of new infections come from people likely unaware they are infectious to others."
According to the CDC, 24% of people who transmit the virus to others never develop symptoms and another 35% were pre-symptomatic. It also said 41% infected others while experiencing symptoms.