During the early days of the coronavirus pandemic, the U.S. Centers for Disease Control and Prevention and World Health Organization made it clear that, unless you're sick or are a medical professional, you do not need to wear a face mask.

On February 29th, U.S. Surgeon General Jerome Adams tweeted: "Seriously people- STOP BUYING MASKS! They are NOT effective in preventing general public from catching #Coronavirus, but if healthcare providers can't get them to care for sick patients, it puts them and our communities at risk!"

Adams' message comes with good intentions. With COVID-19 cases soaring, doctors, nurses and other frontline health-care workers confront a severe shortage of masks — and cautioning people against buying them can help offset the problem.

But other countries have already taken aggressive measures to prevent the spread of COVID-19 by telling citizens to wear masks, even if the masks are homemade.

Last week, George Gao, director-general of the Chinese Center for Disease Control and Prevention, told ScienceMag.com that the "big mistake in the U.S." is that people aren't wearing masks. "This virus is transmitted by droplets and close contact. Droplets play a very important role," he said. You've got to wear a mask, because when you speak, there are always droplets coming out of your mouth."

Gao has done significant research on viruses that have fragile lipid membranes called envelopes — a group that includes SARS-CoV-2 (the virus that causes COVID-19) — and how they enter cells and move between species.

"Many people have asymptomatic or presymptomatic infections," he continued. "If they're wearing face masks, it can [help] prevent droplets that carry the virus from escaping and infecting others."