Up to 40 percent of infected people "seem to have no symptoms," National Institutes of Health (NIH) Director Francis Collins said.
"This might potentially explain why some people seem to fend off the virus and may be less susceptible to becoming severely ill," Collins wrote in a blog post.
Collins said it's "becoming apparent that our country is entering a new and troubling phase of the pandemic as SARS-CoV-2, the novel coronavirus that causes COVID-19, continues to spread across many states and reaches into both urban and rural communities."
The fact that so many who are infected show no symptoms is contributing to the spread, said Collins, who works closely with Dr. Anthony Fauci, the director of the NIH's National Institute of Allergy and Infectious Diseases since 1984.
"They can pass the virus quickly and unsuspectingly to friends and family members who might be more vulnerable to becoming seriously ill," Collins wrote. "That’s why we should all be wearing masks when we go out of the house — none of us can be sure we’re not that asymptomatic carrier of the virus."
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