NEW YORK (CNN) — First, health officials said we shouldn't wear face masks. Then, they said we should. Now, many are saying we must wear masks if we want to protect the economy, reopen more schools and save tens of thousands of lives.

"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But President Donald Trump said he won't consider a federal face mask mandate, citing outdated statements made several months ago.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 health care workers who were quickly overwhelmed with Covid-19 patients. Some medical workers ended up using plastic office report covers or other ordinary items as makeshift, often substandard face masks while working closely with Covid-19 patients.

In March, both the World Health Organization and the CDC said face masks shouldn't be worn by healthy members of the general public and instead should be reserved for those who are sick or caring for the sick.

Adams posted a tweet on February 29 urging people not to buy masks.

But in early April, both the CDC and Adams reversed their guidance after doctors learned more about how easily the coronavirus spreads without symptoms. They even created a video showing how to make a mask with just a T-shirt and rubber bands — no sewing required.

WHO in June also reversed its guidance. The global health agency now says countries should urge the public to wear fabric masks where there's widespread transmission of the virus and where physical distancing is difficult.

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What do doctors say about face masks now?

"People need to know that wearing masks can reduce transmission of the virus by as much as 50%, and those who refuse are putting their lives, their families, their friends, and their communities at risk," said Dr. Christopher Murray, director of the Institute for Health Metrics and Evaluation.

There are just a few groups of people who shouldn't wear cloth masks, including children under 2 years old and people who have breathing problems, according to the Mayo Clinic.

Fauci, nation's top infectious disease expert, is strongly urging the public to wear face masks -- especially now that states have reopened and more people are out in public.

"Plan A: Don't go in a crowd. Plan B: If you do, make sure you wear a mask," Fauci said.

As for the notion that wearing a mask might cause health problems, Fauci said that's not true.

"There has not been any indication that putting a mask on and wearing a mask for a considerable period of time has any deleterious effects on oxygen exchange or anything like that," Fauci said. "Not at all."

Which states have mask requirements?

More than two dozen states and the District of Columbia have mandates on wearing cloth face masks in public.

"It's an incredibly simple, cheap, and now turning out to be effective intervention — both for individuals (and) for communities," Murray said.

"There's no reason that every state shouldn't do what other states are starting to do, which is mandate mask use."

Other countries have had mask requirements nationwide, including Germany, the Czech Republic and Austria.

Do mask requirements actually help?

An estimated 230,000 to 450,000 Covid-19 cases were prevented in the states that enacted requirements for mask use between April 8 and May 15, researchers from the University of Iowa found.

The IHME projected a total of 179,106 US deaths from Covid-19 by October 1. But the number of expected deaths drops by ‭33,059‬ — down to 146,047 — if 95% of Americans would wear masks in public, the researchers said.

What about my civil liberties?

Wearing a face mask will give you more freedom, not less, the surgeon general said.

"Some feel face coverings infringe on their freedom of choice — but if more wear them, we'll have MORE freedom to go out," tweeted Adams, who was nominated by Trump.

"Face coverings → less asymptomatic viral spread → more places open, and sooner! Exercise and promote your freedom by choosing to wear a face covering!"

Murray agreed that one of the fastest ways to help get Covid-19 under control and help prevent a second wave of economic shutdowns is to wear a mask.

"For pretty much every state that we've looked at, if we can get people to wear masks, we can not only save lives, but ... we can also save the economy, because we can keep businesses going."

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Can I stop taking other precautions when I'm wearing a mask?

No. It's still important to keep social distancing when possible and wash your hands frequently — especially because since you could touch a contaminated surface (like the front of your mask) and then touch your face when your mask is off.

"Don't get a false sense of security that that mask is protecting you exclusively from getting infected," said Dr. Deborah Birx, coordinator of the White House coronavirus task force.

"There are other ways that you can get infected."

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