Melissa Leaston Director of nursing at Whittier Street Health Center swabs Steve Rose of Boston at a COVID testing site in Nubian Square on October 15, 2020 in Roxbury, Massachusetts.Matt Stone | MediaNews Group | Boston Herald via Getty ImagesThe United States reported more than 69,000 new coronavirus cases on Friday, the highest daily count the nation has reported since late July.The U.S. has now reported more than 8 million Covid-19 cases and at least 218,600 deaths since the beginning of the pandemic, according to data compiled by Johns Hopkins University.The surge in coronavirus cases comes as infectious disease experts warn the U.S. could face a “substantial third wave” of infections that will be further complicated this winter by the spread of seasonal influenza, which causes many similar symptoms to that of the coronavirus.As colder temperatures arrive in the Northern Hemisphere, more people will spend time indoors and likely fail to follow public health guidance, which creates a greater risk for the cornoavirus’ spread compared with outdoor activities, Dr. William Schaffner, an epidemiologist at Vanderbilt University, said.The U.S. is averaging roughly 55,000 new coronavirus cases every day, based on a weekly average to smooth out the reporting, a more than 16% increase compared with a week ago, according to a CNBC analysis of Johns Hopkins data. New cases were growing by 5% or more in 38 states as the number of infections in the Midwest continues to surge.”We need to pay more attention to this. We seem to forget that we’re making progress, we’re doing better, and then we kind of let go and we go back again,” Dr. Carlos del Rio, a professor at the Emory University School of Medicine who specializes in infectious diseases, told CNBC on Friday.Dr. Anthony Fauci, the nation’s leading infectious disease expert, has warned for weeks that the daily number of new cases has remained “unacceptably high” heading into the end of the year. However, it’s not too late to “vigorously apply” recommended public health measures, such as wearing a mask and maintaining a physical distance from others, Fauci told Johns Hopkins University on Thursday.When the U.S. descended from its first peak in April, 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. 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.— CNBC’s Will Feuer and Nate Rattner contributed to this report.