Dr. Anthony Fauci said coronavirus cases could grow to 100,000 a day in the U.S. if Americans don’t start following public health recommendations.

The nation’s leading infectious disease expert made the remark at a Senate hearing on reopening schools and workplaces.

Asked to forecast the outcome of recent surges in some states, Fauci said he can’t make an accurate prediction but believes it will be “very disturbing.”

“We are now having 40-plus-thousand new cases a day. I would not be surprised if we go up to to 100,000 a day if this does not turn around, and so I am very concerned,” said Fauci, infectious disease chief at the National Institutes of Health.

Fauci said areas seeing recent outbreaks are putting the entire nation at risk, including areas that have made progress in reducing COVID-19 cases. He cited recent video footage of people socializing in crowds, often without masks, and otherwise ignoring safety guidelines.