Across the country, communities including Snohomish County, Wash., Jackson, Miss., and Baton Rouge, La., have seen coronavirus numbers fall and then shoot back up — not unlike the two ends of a seesaw.

In Illinois, Gov. J.B. Pritzker sounded an unusually somber note this past week as he delivered a warning that reverberated across the state: Even though Illinoisans had battled an early flood of coronavirus infections and then managed to reduce the virus’s spread, their successes were fleeting. As of Thursday, the state was averaging more than 1,400 cases a day, up from about 800 at the start of July.

“We’re at a danger point,” Mr. Pritzker said in Peoria County, where the total number of cases has doubled in the last month.

Gone is any sense that the country may soon gain control of the pandemic. Instead, the seven-day average for new infections hovered around 65,000 for two weeks. Progress in some states has been mostly offset by growing outbreaks in parts of the South and the Midwest.