Here's why new cases of the coronavirus are down across most of the U.S.

• White House coronavirus advisor Dr. Anthony Fauci warned members of Congress in late June that the U.S. might report more than 100,000 daily new cases of the virus "if this does not turn around."
• But months later, Fauci's worst fears have not come to pass as daily new cases have steadily fallen across much of the U.S. over the past month.
• Epidemiologists say more unified health messaging, public compliance and closures of businesses are largely to thank for the drop.

EMS medics with the Houston Fire Department move a patient with Covid-19 symptoms onto a stretcher before transporting him to a hospital on August 14, 2020 in Houston, Texas.

John Moore | Getty Images

White House coronavirus advisor Dr. Anthony Fauci warned members of Congress in late June that the U.S. might report more than 100,000 daily new cases of the virus "if this does not turn around."

But months later, Fauci's worst fears have not come to pass as daily new cases have steadily fallen across much of the U.S. While testing has declined in recent weeks, the number of new cases is falling faster than testing rates, indicating that at least some of the drop is real.

Epidemiologists credit a more unified health message in the U.S. that has more people following social distancing rules. They also say that keeping some businesses closed has helped slow the outbreak. And President Donald Trump started endorsing masks in late July, bringing the White House in line with recommendations from the Centers for Disease Control and Prevention after months of resistance.

The number of daily new cases in the U.S. peaked on July 22 at about 70,000 new infections and have steadily fallen to about 42,600 per day, based on a seven-day average, according to a CNBC analysis of data compiled by Johns Hopkins University. The percent of all tests coming back positive has also steadily fallen, from a high of 8.5% in late July to 6.2% this week, according to Hopkins data. That, along with the four weeks of sustained decline and the falling number of Covid-19 hospitalizations, has epidemiologists feeling more confident that the U.S. is finally getting a grip on its outbreak.

"The current plan — wearing a mask, watching your distance, washing your hands, supplemented by smart testing, according to the state plans, surge testing and extreme technical assistance by CDC as well as our craft teams — continues to yield results," Assistant Secretary for Health Adm. Brett Giroir told reporters on a conference call last week.

But the country remains in a delicate spot, according to epidemiologists from some of the hardest-hit states in the country — Florida, Texas and Arizona. While new cases are falling by at least 5% in 31 states, they are still rising by at least as much in more than a dozen states, based on a seven-day average, according to Hopkins data.

Texas

In Texas, new cases are down from an average of about 10,400 on July 22 to about 5,500. While irregularities in the state data led some local health officials and epidemiologists to question the data, Catherine Troisi, an infectious disease epidemiologist at UTHealth School of Public Health in Houston, said the state data is "with a grain of salt, looking good."

She added that the Texas Gov. Greg Abbott's July 2 mask mandate "certainly" helped reduce the spread as well as the closing of bars, "because we know that those were spreading infection." Triosi, however, said that as the number of new cases continues to drop and as society reopens that people may experience "pandemic fatigue" and begin to disregard some guidelines.

"What we're really concerned about are schools opening, Labor Day weekend and pandemic fatigue," she said. "We were all tired of this five months ago. Now we're really tired, so if people see the number of cases come down, they might think it's OK to do things that are a little bit riskier."

"I would not be surprised if we see an uptick after Labor Day," she added.

Florida

In Florida, the daily average number of new cases has fallen from about 11,100 on July 22 to about 3,900 this week. Cindy Prins, an epidemiologist at the University of Florida, attributed much of the drop to changing behavior across the state, prompted by news coverage and effective public health messaging. 

"I think it kind of got to people that opening up didn't mean going back to normal," she said. "I think it got very much in the public eye that we weren't where we needed to be and that we had to take better control."

Prins echoed Troisi's concern that as new cases continue to fall, people might get comfortable and ease up on their commitment to the public health guidance.

"My concern is that we'll have people kind of falling away from this perception of threat as our cases decline and that we could wind up with a little bit of a roller coaster," she said.

However, she added that Florida and many other Sun Belt states hit hard by the virus this summer have an advantage over the northern parts of the country: comparatively warmer weather. Various studies have shown that the virus spreads more easily in crowded, indoor, poorly ventilated environments. Officials, including New York Gov. Andrew Cuomo, have warned that colder weather could present a new challenge as outdoor dining and gatherings become more difficult.

Arizona

Arizona has reported an average of about 650 new cases per day over the past week, down from about 2,750 on July 22, according to Hopkins data. Will Humble, executive director of the Arizona Public Health Association and former director of the Arizona Department of Health Services, attributed the drop to "three core things."

First, he said, Gov. Doug Ducey "finally" allowed towns, cities and counties to implement their own mask mandates in mid-June. Before that, Ducey had prevented local officials from implementing their own face mask requirements. 

"We definitely know that was a huge factor," Humble said.

He added that the following week, Ducey ordered the closure of bars and nightclubs, which he said was another major cause in reversing the direction of the outbreak. 

The third factor that could be driving the drop is that enough of the population might have been infected earlier in the outbreak, giving the virus "a harder time finding new hosts," he said. Particularly hard-hit parts of Arizona might be benefiting from some level of herd immunity, he said, adding that not nearly enough of the population has been infected to actually halt the spread of the virus.

"Arizona adopted the Swedish model," he said. "We threw our hands up and said, 'alright, let's get it.' That's what happened. And we did it. And a lot of people died. And a lot of people recovered. And those people are now protected."

Humble added that he's nervous about the weeks and months ahead, both for the country and for Arizona. He said that under the governor's plan, bars and nightclubs could reopen as soon as this week in some communities. But he said he's not satisfied with the state's plan to ensure compliance with capacity restrictions and other requirements. 

He said that if bars and clubs reopen without a proper enforcement mechanism, it could lead to a major resurgence and "June 2.0." 

"When the bars and nightclubs open with no mitigation measures in place because there was no compliance, who's going to suffer from that? The parents and the kids who could have benefited from in-person instruction," he said, "because it's going to move the metrics up, and then kids are going to have to go back online and in-person instruction won't be an option anymore."

Trouble in the heartland

Director of the Centers for Disease Control and Prevention Dr. Robert Redfield warned last week that while cases are falling nationally, driven by the former hot spots, there are some worrying signs in the middle of the country.

"We're starting to see some of the cases now in the red zone areas are falling, but if you look at those states that are in what we call the yellow zone, between 5% and 10%, they're not falling, so middle America right now is getting stuck," Redfield said in an interview with Dr. Howard Bauchner of the Journal of the American Medical Association. "This is why it's so important for middle America to recognize the mitigation steps that we talked about, about masks, about social distancing, hand washing, closing bars, being smart about crowds."

He said cases are not rising substantially in the region, but the fact that cases appear to be plateauing could be cause for concern, especially as the country enters the colder months of the year and seasonal influenza spreads. Redfield has repeatedly warned that the confluence of a major flu outbreak along with the Covid-19 pandemic could overwhelm hospitals and cause preventable deaths. He encouraged Americans to get this year's flu vaccine as soon as it's available to mitigate the risk of an overwhelming flu season.

"We don't need to have a third wave in the heartland right now," he said. "We need to prevent that particularly as we're coming to the fall."

— CNBC's Nate Rattner contributed to this report.