More than 1% of the United States population has tested positive for coronavirus, but various factors have led experts to believe millions more Americans have contracted the virus.

According to Johns Hopkins University, there are 3.30 million confirmed COVID-19 cases in the U.S. as of Monday, July 13. The country’s census bureau reports the current population is 329.9 million.

But how many Americans have really had coronavirus? The real number is difficult to determine, but dozens of states may be undercounting how many of their residents have had the virus.

Despite guidance from the U.S. Centers for Disease Control and Prevention, 28 states were not reporting probable cases, CNN reported last month.

CDC Director Dr. Robert Redfield said June 25 “there actually were 10 other infections” for each reported case, NPR reported. He estimated between 5% to 8% of the American population had been exposed to the virus, according to NPR.

Researchers at Massachusetts Institute of Technology said that by June 18, around 17.5 million people in the country had coronavirus at one point, Business Insider reported.

In a study published on Science Translational Medicine in June, researchers determined there were more than 8.7 million COVID-19 cases during a three-week stretch in March. There were only 100,000 confirmed cases during that same time frame, according to Business Insider.

Similarly, the number of deaths due to coronavirus represents “a substantial undercount of the true burden,” Yale epidemiologist Dan Weinberger told CNBC.

So why are numbers being so drastically undercounted?

Redfield’s estimate that up to 24 million Americans have had coronavirus was based on antibody tests, according to the Washington Post.

Many people are asymptomatic and for that reason have not been tested for the disease.

“It definitely tells us there are more infections out there that either asymptomatic or so mild that people are not seeking testing. A lot of the disease goes unreported,” Heather Kerwin, the epidemiology program manager at the Washoe County, Nevada health department, told the Associated Press.

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A study published this month in the Journal of the American Medical Association states only 10% to 15% of cases in the U.S. were being confirmed in the early part of the pandemic.

The U.S. is now testing more people, but the infection rate is also rising, according to the New York Times.

A Houston infectious disease expert told KENS “every American will know someone hospitalized with COVID-19.”

“We are in a full-blown public health crisis right now,” Dr. Peter Hotez told the station. “These aren’t hotspots. This is widespread devastation.”