An outbreak of covid-19 cases has been reported at 15 fraternity houses at the University of Washington in Seattle, underscoring the risk that college students could face if they return to campus during the pandemic and fail to follow rules about wearing masks and social distancing.

The Interfraternity Council, a student-led governing board for fraternities at the university, reported on Friday that at least 117 residents currently living in 15 fraternity houses have self-reported testing positive for covid, the university said.

School officials said they are still collecting and verifying the status of cases — and that there may be more than what has been reported.

The Seattle Times quoted Jeff Duchin, a public health official for Seattle and King County, as saying the outbreak is “very concerning.”

“It does make us worry about what might happen this fall,” Duchin told the paper. “We understand there was a lot of socializing, a lot of risk-taking, not a lot of mask-wearing going on. … When you do that and you don’t take precautions, you get a lot of covid.”

The outbreak at the fraternities comes as colleges and universities around the country are finalizing fall plans, with many of them opting to allow students on campus, in some cases taking some or even all of their courses online.

Concerns are rising about plans to let students back on campus in some parts of the country, including in California, as covid-19 cases are spiking.

The University of Southern California announced this week that it was pulling back on plans to welcome students to campus, saying it would offer mostly virtual learning and urging students to find off-campus housing. In a July 1 memo, officials wrote:

Public health guidelines continue to change, and Los Angeles County has yet to approve our plans for returning to full campus operations. Los Angeles is experiencing an alarming spike in coronavirus cases, making it clear we need to dramatically reduce our on-campus density and all indoor activities for the fall semester … Given the continuing safety restrictions and limited densities permissible on campus, our undergraduate students primarily or exclusively will be taking their courses online in the fall term, and on-campus housing and activities will be limited. While not what we hoped, we are now recommending all undergraduates take their courses online, and reconsider living on or close to campus this semester. We are continuing with limited in-person, on-campus activity because we believe we can keep students, researchers, staff, and faculty safe with our low-density plan.

At the University of Arizona, which had already announced it was reopening campus for in-person classes this fall, President Robert Robbins said last week that the plans may have to change because a spike in covid-19 cases in the state has filled the intensive care units in hospitals.

“If I had to say today would we would reopen, no, because … the ICUs are full,” he said during a weekly update on the university’s plans. “We cannot have a situation where we’re bringing students back to campus, asking our faculty and staff to come back to campus when we’re in truly an exponential growth of the number of cases here.”

University of Washington President Ana Mari Cauce said in a June 29 letter to new and returning students that the school now plans for students to return to campus in September and for classes to start on Sept. 30 with a hybrid model — both in-person and virtual learning. She wrote:

We are offering as many in-person courses as possible, prioritizing hands-on courses, such as studio, clinical and lab courses, and courses for first-year undergraduate and graduate students … To maintain a safe six feet of physical distance among students and instructors, small classes will meet in big rooms. Large classes — including all courses of 50 students or more — will be taught remotely. Depending on your area of study and where you are in your academic career, you may have many of your classes in person or very few. Or you may attend the lecture portion of your class online and then gather in person for discussion group sections with your classmates. To allow for appropriately distanced passing between classes, we will not schedule courses back to back where physical distancing is impossible because of narrow hallways and limited queuing spaces.