Science has become much more advanced since then. Prather and others have developed instruments that can “see” very tiny particles -- the aerosols. Aerosols are measured in microns, or one one-millionth of a meter. A human red blood cell is about 5 microns in size. These particles are so small that the moisture from a cough or sneeze evaporates while they’re still in the air. They float on air currents. It takes them hours to settle.

Aerosols, she writes, “can accumulate, remain infectious in indoor air for hours, and be easily inhaled deeply into the lungs.”

Prather realizes this is a scary thing to be telling people. “I hesitate. I don’t want to freak people out.”

She also believes knowledge is power.

“I have to say something because this could actually protect people,” she says. What airborne transmission means, she says, is that 6 feet is not far enough to stand apart. It also means we should probably be wearing masks a lot more often.

“It’s very fixable. Masks aren’t that big of a deal,” she says. “To me, I look at this as a solution.”

She points to the success Taiwan has had stopping the spread of the infection. Taiwan has only had a few hundred cases and only seven deaths, even though the country never implemented a national lockdown. Instead, they aggressively tested their citizens, quarantined people who tested positive for 2 weeks, and had everyone wear face masks.

“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.