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Something has been bothering Kimberly Prather, PhD: Everything she reads about COVID-19 points to a pathogen that travels through the air.

There’s how quickly it has spread around the world, studies showing how it spreads through restaurants (maybe by the air conditioning system?), how it attacked a church choir even though they were spread apart while they were singing, how it seems to spread like wildfire on planes and on cruise ships; all of this, she says, Prather should know. She studies aerosols -- particles so tiny they float freely through the air, traveling feet or even miles. She runs a large, government-funded research center at the University of California San Diego to study how viruses and other things that come out of the ocean float through the air.

“A lot of the evidence has been pointing to aerosol transmission of respiratory viruses,” she says. Influenza can be passed through the air, as can the virus that causes SARS. “This particular virus, a lot of evidence is mounting.”

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“If you look at countries that just naturally wear masks when people feel sick … those countries did a lot better than those that did not,” Prather says.

She wears hers inside and outside, especially if she’s walking outdoors in a place where she can see other people. She tells people to imagine how far they can smell cigarette smoke or a barbecue. That’s how far aerosols can travel between you and another person.

The good news, she says, is that recent studies have shown that homemade cloth masks can be as effective at blocking the virus as surgical masks. There’s one big caveat, though. They have to fit to your face.

“If you look at all these people who are wearing bandanas, they’re just hanging down. That’s not good because aerosols will just flow right around,” she says. “Aerosols are really light. If you can feel a breeze, they will be in that breeze.”