"If we all wore face coverings for the next four, six, eight, 12 weeks across the nation, this virus transmission would stop," said Dr. Robert Redfield, director of the Centers for Disease Control and Prevention.

But President Donald Trump said he won't consider a federal face mask mandate, citing outdated statements made several months ago.

"Dr. Fauci said don't wear a mask. Our surgeon general, terrific guy, said don't wear a mask. Everybody was saying don't wear a mask," Trump told Fox News' Chris Wallace in an interview that aired Sunday.

"All of a sudden, everybody's got to wear a mask. And as you know, masks cause problems too. With that being said, I'm a believer in masks. I think masks are good."

It's true that back in February and March, health officials such as Dr. Anthony Fauci and US Surgeon General Dr. Jerome Adams advised the general public not to wear face masks.

But for the past several months, Fauci, Adams, the CDC and the World Health Organization have all agreed that wearing face masks is critical in stopping the spread of the coronavirus. That's because doctors and scientists now know much more about how easily the virus spreads:

-- It's easy to spread this virus by just talking or breathing.

-- This coronavirus is highly contagious. Without mitigation efforts like stay-at-home orders, each person with the coronavirus infects an average of two to three other people. That makes it twice as contagious as the flu.

-- It's easy to spread the coronavirus without any symptoms -- either from asymptomatic carriers or pre-symptomatic carriers.

-- This virus has a long incubation period -- up to 14 days -- giving a wide window of opportunity for people to infect others before they even know they're infected.

-- Carriers may be most contagious in the 48 hours before they get symptoms, making transmission even more blind.

In other words, it's not just people who are sneezing and coughing who can spread coronavirus. It's often people who look completely normal and don't have a fever. And that could include you.

"Cloth face coverings are meant to protect other people in case the wearer is unknowingly infected but does not have symptoms," the CDC said.

It said everyone "should wear a cloth face cover when they have to go out in public, for example to the grocery store or to pick up other necessities."

But the benefits go both ways. "Your cloth face covering may protect them. Their cloth face covering may protect you," the CDC said.

If 95% of Americans wore face masks in public, it could prevent 33,000 deaths by October 1, according to the University of Washington's Institute for Health Metrics and Evaluation.

Why did health officials tell us not to wear masks before?

Early in this pandemic, scientists didn't know how easily this virus spreads between people without symptoms or how long infectious particles could linger in the air.

There was also a shortage of N95 respirators and face masks among