Colder temperatures are arriving in the Northern Hemisphere, and an insidious rise in new coronavirus cases in the U.S. and Europe is underway.
For months, health officials have warned against this possibility, and as these trends begin to materialize, countries are weighing whether to impose stricter measures to contain the virus' spread.
"Our worry has been that we would see a fall wave, that we'd see a big resurgence in the fall," said Justin Lessler, an associate professor of epidemiology at the Johns Hopkins Bloomberg School of Public Health. "And that has really been something I think all of us in the public health community have been worried about for a while."
In the U.S., coronavirus cases were growing by 5% or more in 38 states, as of Friday, according to a CNBC analysis of data compiled by Johns Hopkins University that uses a weekly average to smooth out the reporting. The nation is averaging roughly 55,000 new coronavirus cases every day, a more than 16% increase compared with a week ago.
"It's still not too late to vigorously apply good public health measures, and again I emphasize without necessarily shutting down the country," White House coronavirus advisor Dr. Anthony Fauci told Johns Hopkins University during a recorded Q&A on Thursday.
Fauci, the nation's leading infectious disease expert, has warned the daily number of new coronavirus cases in the U.S. remains dangerously high, especially as the forthcoming flu season threatens to complicate the nation's response to the pandemic.
When the U.S. descended from its first peak in April, where cases were largely driven by New York and other states in the Northeast, the number of new coronavirus cases "got stuck" around 20,000 per day, Fauci said. Ideally, the U.S. would've reported less than 10,000 cases every day, he said.
Then cases resurged in America's Sun Belt over the summer as states tried to reopen their economies. The number of daily new Covid-19 cases swelled to a high of nearly 70,000 cases a day before subsiding once again. However, new cases have since hovered between 40,000 to 50,000 cases a day.
"You can't enter into the cool months of the fall and the cold months of the winter with a high community infection baseline," Fauci said. He added that the positivity rate, or the percentage of tests that are positive, is "going in the wrong direction" in more than 30 states.
"I'm pretty glum at the moment because it does look as though in the majority of states there's an increasing number of cases," Dr. William Schaffner, an epidemiologist at Vanderbilt University, said. "There's a growing sense of coronavirus fatigue out there. People really want to get back to the old normal."
Schaffner said he expects the U.S. to experience a "substantial third wave" of infections and that it will be further complicated this winter by the spread of seasonal influenza, which causes many similar symptoms to that of the coronavirus.
People will spend more time indoors and likely fail to follow public health guidance as the colder temperatures arrive, which creates a greater risk for the cornoavirus' spread compared with outdoor activities, Schaffner said.
"During the summer, people went indoors for air conditioning, but they did spend more of their time outdoors. Nonetheless, it spread as people became lax in their attention to social distancing and mask wearing," he said. "As far as I can tell, that's growing."
Unlike previous outbreaks across the U.S., the coronavirus is now widely circulating through many of America's more rural communities.
North Dakota, South Dakota, Montana and Wisconsin continue to report the highest number of cases per 100,000 residents in the country, according to Johns Hopkins data.
The outbreak has turned for the worst in some states. Wisconsin Gov. Tony Evers activated an alternative care facility that opened to overflow coronavirus patients this week at the Wisconsin State Fair Park.
In North Dakota, where cases have grown nearly 34% compared with a week ago, local officials are concerned that some hospitals may not have the staffing required to treat severely ill coronavirus patients.
"We have had a nursing shortage prior to the pandemic, and so the additional workload and additional hospital capacity that's come with Covid has impacted and affected staffing," said Renae Moch, director of Bismarck-Burleigh Public Health.
Moch said that at times, some residents of more rural parts of North Dakota have had to travel hours across state lines to Montana and South Dakota to seek care because hospital beds in Bismarck, the state's capital, were full.
"For us, this is the worst that it's been," Moch said. "I think especially with the flu season coming up and the possible impacts of that as we move into the fall ... we need to get a handle on this before it gets any worse, and I'm sure it can get worse."
Dr. Allison Suttle, the chief medical officer at Sanford Health, which has hospitals located in North Dakota and South Dakota, said they've seen more Covid-19 patients and have added beds to help treat people.
Because the current surge of coronavirus patients is happening now versus earlier in the nation's response to the pandemic, Sanford Health has had time to prepare and stock up on needed supplies, Suttle said, adding that she's confident they can treat patients who seek medical care.
However, people in more rural states delayed their routine health-care visits in the spring after they were told to stay home as the coronavirus swept through the coasts. Now, those people are coming into the hospitals in worse condition amid the growing number of Covid-19 patients, Suttle said.
"What we're seeing as the hospitalizations of Covid increase in direct correlation to the number of cases increasing in our communities, we're also seeing sicker patients that have delayed care in March and April that are now coming in with problems," she said. "They require more intense care, longer hospital stays, so that all compounds."
The United States is not the only country reporting climbing new cases.
When adjusting for population, the number of new coronavirus infections in Europe has now overtaken the United States, with Europe reporting 187 new cases per million people, based on a seven-day average, compared with 162 new cases per million people in the U.S. as of Thursday.
Europe — which in CNBC's analysis of Hopkins data includes the 27 European Union countries plus the United Kingdom — is reporting an average of roughly 97,000 new cases per day, up 44% from one week ago.
The World Health Organization warned on Friday that Europe's outbreak is "concerning" as the number of intensive care unit beds in some regions decline. Maria van Kerkhove, the WHO's technical lead, said the organization knows "of a number of cities across Europe where ICU capacity will be reached in the coming weeks."
The virus' resurgence prompted France to declare a public health state of emergency. The U.K. placed tighter restrictions on gathering and indoor business operations in London, and threatened to take more action nationwide if necessary. Germany also imposed new measures to curb the virus' spread.
Meanwhile, Canada is experiencing a second wave of coronavirus infections as the provinces of Quebec and Ontario report the bulk of the country's Covid-19 death toll, Carissa Etienne, WHO's regional director for the Americas, said on Wednesday.
— CNBC's Berkeley Lovelace Jr. contributed to this report.