A little more than three weeks ago, officials in Pittsburgh announced a milestone enviable for almost any major city in America: A day had gone by without a single new confirmed case of the coronavirus. It was good news for a city that had seen only a modest outbreak all along, even as the virus raged through places like Philadelphia and New York.

That was then.

Western Pennsylvania is suddenly experiencing an alarming surge of infections. Allegheny County, which includes Pittsburgh, reported more than 100 new cases for the first time on June 30; two days later, the daily case count surpassed 200. Over two weeks in late June and early July, the county recorded more new cases than in the previous two months combined, and on some recent days has accounted for nearly half of all new known cases in Pennsylvania.

“Allegheny County is the big area of concern at this point,” Gov. Tom Wolf said at a news conference last week. “There have been others more modest,” he said, “but right now Allegheny County is the area.”

The spike in the Pittsburgh area offers a cautionary tale: Even after months of vigilance, an outbreak can flare up all of a sudden. While the nation’s current flood of new cases is being driven primarily by the spread of the coronavirus in the South and the West, experts fear that other parts of the country — including places like Cleveland, Milwaukee, and Kansas City, Mo., which are all seeing new growth — could be close behind.

“You are seeing what could be the beginning of what we’ve been seeing in Texas and Arizona,” said Dr. Bill Miller, a professor of epidemiology at the Ohio State University. He described upswings in urban counties in Ohio, a state that saw weeks of steady or declining cases but is now averaging more than 1,000 new confirmed cases a day, the worst so far of the pandemic.

“We can’t let our guard down,” he said.

For months, Pittsburgh had been both diligent and lucky.

The virus began spreading here later than in some early centers of the nation’s crisis, like New York City or Detroit. That gave Pittsburgh time to prepare. At the same time, Pennsylvania, which began facing skyrocketing rates in the eastern half of the state, took a more aggressive approach to shutting down public life than states like Florida and Texas, which closed later and reopened earlier.

Pittsburgh, which has an economy driven by the health care industry and is a sister city to Wuhan, China, where the coronavirus first emerged, took the threat seriously. Its 300,000 residents largely abided by the new way of life, ordering their pizzas from Mineo’s to go, drinking their Yuenglings on the porch at home and wearing masks for grocery trips to Giant Eagle, even as the case numbers remained relatively low.

From March 23, when the governor ordered everyone to stay at home, until June 5, when Allegheny County was allowed to lift the more stringent restrictions, the city had hunkered down. But it was not long after that limited reopening in June, as people flocked to bars for the first time in months, that the seeds of the current surge were planted.“You have to realize: The virus isn’t going to go anywhere until there is a vaccine,” said Dr. Amesh Adalja, a Pittsburgh-based physician and a senior scholar at the Johns Hopkins University Center for Health Security. “You are going to see these flare-ups in any city, because wherever there are people, there is this virus.”