MUNCIE, Ind. (AP) — Just two weeks after students started returning to Ball State University last month, the surrounding county had become Indiana’s coronavirus epicenter.

Out of nearly 600 students tested for the virus, more than half have been positive. Dozens of infections have been blamed on off-campus parties, prompting university officials to admonish students.

University President Geoffrey Mearns wrote that the cases apparently were tied not to classrooms or dormitories but to “poor personal choices some students are making, primarily off campus.”

“The actions of these students are putting our planned on-campus instruction and activities at risk,” he said.

Similar examples abound in other college towns across the nation. Among the 50 U.S. counties with the highest concentrations of students and overall populations of at least 50,000, 20 have consistently reported higher rates of new virus cases than their states have since Sept. 1, according to an Associated Press analysis.

On average, infection rates in those 20 counties have been more than three times higher than their states’ overall rates.

At James Madison University in Virginia, which recently sent students home through September amid a surge in cases, the county is averaging a weekly infection rate of nearly 90 cases per 100,000 people, or more than eight times the statewide average.

Health officials fear that surges among college students will spread to more vulnerable people — older ones and those with underlying health problems — and trigger a new wave of cases and hospitalizations. Some worry that colleges could overwhelm hospitals already bracing for increasing cases of COVID-19 and flu this fall and winter.

“There’s this waiting game. Does it stay on college campuses or will it escape?” said Dr. Jeff Pothof, chief quality officer at the University of Wisconsin medical center in Madison, where cases among college students have been climbing.

While universities have emerged as hot spots in nearly every state, many of the worst outbreaks have been scattered across the South and Midwest. Of the 50 college counties analyzed by the AP, James Madison’s had the highest infection rate, followed by counties that are home to the University of Georgia, Florida State and Indiana University in Bloomington.

In the 10 counties with the highest infection rates, colleges have reported at least 15,000 cases among students and employees in recent weeks, though testing and reporting practices vary significantly and the actual number is probably much higher.

For many colleges, the return to campus was a carefully orchestrated process that took months to plan and millions of dollars to pull off. But as safe as they’ve made their campuses, many colleges have struggled to curb off-campus gatherings that have been tied to thousands of infections.

Parties were blamed for dozens of cases at the University of North Carolina at Chapel Hill, which brought students back in early August only to send them home weeks later.

Other schools have cracked down on parties and disciplined students. The University of Missouri in Columbia announced this week that it expelled two students and suspended three others for violating rules meant to slow the virus’s spread.