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A mantra has emerged among health professionals calling for aggressive action on the coronavirus outbreak: "Flatten the curve."

The catchy phrase refers to a so-called epidemic curve that is commonly used to visualize responses to disease outbreaks — and illustrates why public and individual efforts to contain the spread of the virus are crucial.

It's all about speed.

Our #FlattenTheCurve graphic is now up on @Wikipedia with proper attribution & a CC-BY-SA licence. Please share far & wide and translate it into any language you can! Details in the thread below. #Covid_19 #COVID2019 #COVID19 #coronavirus Thanks to @XTOTL & @TheSpinoffTV pic.twitter.com/BQop7yWu1Q — Dr Siouxsie Wiles (@SiouxsieW) March 10, 2020

While the virus can't be stopped in its tracks, it can be slowed. Stopping it from spreading quickly will help ensure that health care systems can cope with the strain of the outbreak.

But if everyone gets sick at once — represented in the chart as the sharp uptick — it's a problem.

Splitting the chart is a line: the finite capacity of the health care system. Above that line, it becomes far more challenging to treat both coronavirus patients and people sick with any other illnesses.

That leads to a need to "flatten the curve," or slow the infection rate, leaving health care systems better placed to treat people — and save lives.

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This is where containment strategies, such as banning large gatherings and encouraging people to limit their exposure to others, come into play and why individual efforts to stop the spread of the virus are crucial.

In other words, by flattening the curve, countries that already can't stop the virus can at least slow its spread so the health care system can adequately take care of people. It's a somewhat counterintuitive notion — that a longer outbreak can be easier to handle.

"If you think of it from the virus' perspective, it would like to spread quickly and to as many people as possible, so it wants a rapid outbreak," said Jodie Dionne-Odom, an assistant professor of medicine in the division of infectious diseases at the University of Alabama at Birmingham. "The reason to flatten out the curve is really to give us time to respond to a rapidly worsening outbreak."

The difference is crucial. In the U.S., where lags in testing have already hobbled efforts to contain the virus, measures to help slow its spread will be particularly important to avoid a huge swell of new infections that could quickly overwhelm hospitals and resources.

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Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, said avoiding sharp spikes in the epidemic curve could reduce the overall number of coronavirus deaths in the country.

"What we need to do is flatten that down," Fauci said Tuesday during a coronavirus task force briefing at the White House. "You do that by trying to interfere with the natural flow of the outbreak."

The "flatten the curve" rallying cry comes as some health professionals are warning that the U.S. is woefully behind other countries in taking precautions to limit the spread of the new coronavirus.

The chart also points to what public health experts have warned could be the biggest risk posed by the coronavirus — that a serious outbreak could overwhelm an already-stretched health care system. The U.S. is dealing with a significant flu season, meaning the breaking line on the chart — the finite capacity of the health care system — is lower than people may think.

"A lot of hospitals already function at capacity, and a high number of flu cases are still being diagnosed in our hospitals, so flattening the curve gives hospitals and public health experts time to plan and prepare," Dionne-Odom said.

The phrase has picked up momentum in recent days, with various versions of the chart circulating on social media, including some that are animated to show how the curve can move depending on how people act.

Some people have rallied around a hashtag: #flatteningthecurve. Climate activist Greta Thunberg pointed to the graph Wednesday, saying, "We can't solve a crisis without treating it as a crisis and we must unite behind experts and science."

Dale Fisher, chairman of the World Health Organization's Global Outbreak Alert and Response Network, referred to the idea in an interview Feb. 26 with the BBC. Since then, the phrase has gained steam.

Fisher said in a telephone interview Wednesday that understanding the importance of the curve is crucial to communicating the need for action. He pointed to Italy, where the outbreak has pushed some hospitals to their breaking points.

"You can be the best hospital in New York or Singapore ... and eventually your capacity will be breached and then you've got people being ventilated in corridors," said Fisher, an infectious diseases doctor who is a professor of medicine at the National University of Singapore.

He said the emergence of the phrase in common use has given him some hope, particularly after a politician in Singapore, where he lives, used it on television.

"I punched the air and I said, 'yes,'" Fisher said. "It's so important for people to understand."