Millions of Americans have probably had the coronavirus without knowing it.

That's the conclusion of officials at the Centers for Disease Control and Prevention and many other experts.

"Our best estimate right now is that for every case that was reported, there actually were 10 other infections," Dr. Robert Redfield, the director of the CDC, said during a call with reporters Thursday.

Redfield estimates that between 5% and 8% of the U.S. population has been exposed. He points to results from communitywide antibody tests and other surveillance measures that point to this range. But, he emphasizes, that leaves more than 90% of Americans who have yet to be exposed and who remain susceptible.

To date, 2.3 million Americans have had confirmed coronavirus infections, but by the CDC's estimates, the real number could be at least 20 million.

The estimates validate what many public health researchers suspected — that the health system has failed to capture much of the spread of the virus within some communities.

"In the beginning, there wasn't a lot of testing that was done of younger, asymptomatic individuals," Redfield said. "So I think it's important for us to realize that we probably recognized about 10% of the outbreak by the methods that we used to diagnose between the March, April and May."

And certain people are more susceptible to serious illness from COVID-19.The CDC points to consistent evidence that a range of conditions put people at a higher risk. These include chronic kidney disease, chronic obstructive pulmonary disease (COPD), Type 2 diabetes, obesity, chronic heart disease, sickle cell disease and being immunocompromised.

Age also plays a role in the risk of serious illness, with most deaths from COVID-19 documented in people 65 and older. And the risk for serious illness or death increases the older a person is.

"While we are all at risk for COVID-19, we need to be aware of who is susceptible to severe complications so that we take appropriate measures to protect their health and well-being," Redfield said. The more risk factors people have, the higher their risk of serious illness or death. In fact, an analysis in the CDC's Morbidity and Mortality Weekly Report, published last week, found that people with chronic conditions were 12 times more likely to die from the virus compared with those who didn't have underlying conditions.

Given that about 60% of American adults have at least one chronic medical condition, it's clear that many are at elevated risk of severe illness from COVID-19. Lifestyle-related diseases including Type 2 diabetes and obesity are among the most common conditions in the United States. These conditions can increase inflammation and tamp down the immune system.

In addition, the CDC published an analysis Thursday pointing to increased risks for pregnant women from COVID-19. Although pregnant women are not at greater risk of death from the virus, the study finds they are more likely to be hospitalized and more likely to be admitted to the intensive care unit. They're also more likely to require intubation for breathing support compared with women who are not pregnant.

Given the risks, Redfield urged Americans to continue to take precautions. "This is still serious. It's significant," he emphasized. Case counts are on the rise in many Sunbelt states, and the virus continues to circulate widely.

"The most powerful tool that we have is social distancing," Redfield said. He also pointed to the importance of face coverings and good hand hygiene. "It's really important. [These are] really powerful tools. And as we go into the fall, in the winter, these are going to be really, really important defense mechanisms."

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