Masks are a critical tool in preventing COVID-19, the CDC said, because symptoms do not appear for several days and people can unknowingly spread the virus.

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People who aren’t showing COVID-19 symptoms but have the virus are causing most of the spread, the Centers for Disease Control said in a new guidance.

In most COVID-19 cases, people do not begin to show symptoms, such as coughing, fever and shortness of breath, for about six days after they are infected. During that time span, people are highly infectious and typically unaware that they have the virus, leading to unintended spread. People can also be asymptomatic and have the virus but never show symptoms.

"Most SARS-CoV-2 infections are spread by people without symptoms," the CDC said. "CDC and others estimate that more than 50% of all infections are transmitted from people who are not exhibiting symptoms. This means at least half of new infections come from people likely unaware they are infectious to others."

This period when people are pre-symptomatic but infectious is why masks are a critically important tool in slowing the spread of COVID-19, the CDC said. Masks — even non-medical cloth versions — benefit other people, as they stop infectious people from spreading the virus, and protect the wearer from taking in others’ respiratory droplets.

“The prevention benefit of masking is derived from the combination of source control and personal protection for the mask wearer,” the CDC said. “Cloth masks not only effectively block most large droplets, but they can also block the exhalation of fine droplets and particles (also often referred to as aerosols) smaller than 10 microns.”

“Upwards of 80% blockage has been achieved in human experiments,” they continued, “… with cloth masks in some studies performing on par with surgical masks as barriers for source control.”

Currently, about half of the U.S. requires people to wear masks in public spaces and enforces some kind mask mandate. President Donald Trump has resisted instituting a nationwide mask mandate and often mocked people who wear masks, including President-elect Joe Biden, despite research showing that a mandate could save 130,000 American lives through the winter season. As Biden prepares for his presidency, he urged all Americans to wear masks.

“Please, I implore you, wear a mask, do it for yourself, do it for your neighbor,” he said on Nov. 9. “A mask is not a political statement, but it is a good way to start pulling the country together.”

Currently, the U.S. is struggling to contain a massive increase in COVID-19 cases. Nearly every state is seeing some of their highest daily case totals of the entire pandemic, and U.S. cases have increased by 2.5 million in November. As of Nov. 23, more than 12,418,700 Americans have tested positive for COVID-19, and at least 257,117 people have died from the virus, according to The New York Times.

The CDC said that “Adopting universal masking policies can help avert future lockdowns, especially if combined with other non-pharmaceutical interventions such as social distancing, hand hygiene, and adequate ventilation.”

The information in this story is accurate as of press time. However, as the situation surrounding COVID-19 continues to evolve, it's possible that some data have changed since publication. While Health is trying to keep our stories as up-to-date as possible, we also encourage readers to stay informed on news and recommendations for their own communities by using the CDCWHO, and their local public health department as resources.

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This story originally appeared on people.com