Wearing a mask and maintaining physical distance from others is important even when none of you are showing symptoms of COVID-19, experts say.

This is partly due to the risk of transmission by asymptomatic people — those who have the novel coronavirus, but aren’t showing any symptoms.

This is playing a role in the spread of COVID-19, the World Health Organization said Tuesday, clarifying some remarks they made just a day before suggesting that it only made a small difference.

“Some estimates of around 40 percent of transmission may be due to asymptomatic (cases), but those are from models. So I didn’t include that in my answer yesterday but wanted to make sure that I made that clear,” said Dr. Maria van Kerkhove, a WHO epidemiologist and technical lead on the pandemic.

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Here’s what we know, and don’t know, about the phenomenon so far.

What is asymptomatic transmission?

Dr. Isaac Bogoch, an infectious diseases specialist at Toronto General Hospital, believes it’s important to draw distinctions between three categories of patients.

These are asymptomatic patients, who have the disease but never show any signs of it,