CHICAGO (WBBM NEWSRADIO) — This year’s aggressive flu season is taking a toll on employers across the country.

If workers take four sick days to recover from the flu, the cost to employers in lost productivity could reach over $9 billion, according to an estimate from global outplacement and executive coaching firm Challenger, Gray & Christmas, Inc.

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“The current strain is particularly aggressive, and those who are sick can be contagious for up to seven days,” Andrew Challenger, the firm’s vice president of global outplacement consultancy, said in a statement. “Workers who have the flu, or need to care for someone with the flu, should absolutely not come into work. This is exactly why employers have sick leave benefits.”

Challenger came up with the $9 billion estimate like this: It took the number of people over 18 who got sick last year, combined it with how many people are employed and the average hourly wage of $26.63. That came to 11 million people. Now, multiply 11 million by four sick days and Challenger’s cost estimate to employers is $9,415,586,823.84.

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“The 2014-2015 flu season was the worst in recent years, according to the CDC. This year’s flu season is on pace to be similar, if not worse. Employers should prepare to see a number of absences in the coming weeks,” Challenger said.

On average, this year’s H3N2 flu virus is taking people out of work for about four days.

“It looks like about 11 million people could be missing four 8-hour shifts this flu season. And that adds up to about $9.4 billion in lost productivity for American employers,” Challenger said.

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Basically, if you think you’re coming down with the flu, here’s the advice: Stay home.