Two Southern states reported their highest daily totals of confirmed coronavirus cases Thursday. 

Alabama confirmed 1,129 new cases, while Mississippi reported 1,092 new cases. On Wednesday, Florida reported its highest daily total of 5,508 cases.

Alabama now has 32,753 cases, while Mississippi is reporting 24,516. On Thursday, Florida’s total stood at 114,018 cases, counting Thursday’s new single-day total of 5,004.

U.S. officials believe as many as 20 million Americans have contracted the coronavirus, suggesting millions had the virus and never knew it.

Twenty million infections would mean about 6% of the nation’s 331 million people have been infected, leaving most of the population still susceptible to the virus. 

Previously, officials at the Atlanta-based Centers for Disease Control and Prevention and the nation’s top infectious disease expert, Dr. Anthony Fauci, have said that as many as 25% of infected people might not have symptoms.

The new estimate is based on CDC studies of blood samples collected nationwide. Many infections were not caught in early testing, when supplies were limited and federal officials prioritized testing for those with symptoms.

Dr. Thomas Tsai, a Harvard University health policy researcher, said 20 million seems reasonable, but “most of these estimates exist in a range” and it’s important to know how wide that is.

“It’s hard to interpret this just from a single number and without the context for it,” such as what locations were sampled and whether it was truly a random slice of a population or areas of low or high prevalence, which can skew the results.

A dozen states in recent weeks have seen a worrisome uptick in new cases, as well as in the more critical measure of the percentage of positive cases discovered in tests performed. Seven states have seen more than 10% of tests come back positive. And troublesome spikes in Sun Belt states have dominated news coverage in recent days.

As states reopen, the Trump administration says it is up to governors and local officials to determine how to respond to the spikes. Texas GOP Gov. Greg Abbott, for example, recommended that most residents remain home for their safety, as his state sees virus infections increasing in major metro areas.

The CDC teams, officials said, are working to trace new outbreaks and reinforce protective measures such as social distancing and mask wearing in hard-hit areas and to remind vulnerable populations to take extreme precautions. The administration says those efforts have helped slow new infections in North Carolina and Alabama, where they were deployed earlier this month.

The officials say the nature of the outbreak now is different than months ago, when deaths topped more than 1,000 per day for weeks and hospitals were stretched beyond capacity across the country. The new increase in positive cases, they said, is capturing what has long been there. They say it is only now showing up in data because the U.S. has increased testing and surveillance.

With testing far more widespread now, officials believe 50% of new cases in Florida and Texas are among people ages 35 or younger, and most of them are asymptomatic.

The U.S. is testing about 500,000 patients per day. On a per capita basis, the U.S. rate falls behind several other countries, including Spain, Australia, Russia and Iceland, according to figures from Johns Hopkins University.

The Associated Press contributed to this report.