Public health officials are investigating an influenza outbreak at a Michigan university that has resulted in more than 500 cases, as several schools have also seen surges in flu activity.

The University of Michigan’s Ann Arbor campus in Washtenaw County has reported over 525 cases among students since Oct. 6 — about three-quarters of them among people unvaccinated against the flu, school officials said this week.

The university said it is tracking a “large and sudden increase” in cases, with nearly all reported in the past two weeks.

“While we often start to see some flu activity now, the size of this outbreak is unusual,” Juan Luis Marquez, medical director at the Washtenaw County Health Department, said in a statement.

A team from the Centers for Disease Control and Prevention is helping the university and local and state health departments investigate the outbreak, including how the flu is spreading.

Amid the surge, the Michigan Department of Health and Human Services is calling on people to get the flu vaccine, as vaccine administrations are down by about 26% compared to this time last year and COVID-19 cases are also on the rise.

“This outbreak comes at a time when COVID-19 infections are again surging in Michigan, with case rates, positivity rates, hospitalizations and deaths all increasing,” the department said in a release Wednesday. “State and local public health officials are concerned with the potential for increased strain on health systems if COVID-19 and influenza cases surge at the same time this winter.”

Other campuses are also battling flu outbreaks. Rowan University in Glassboro, New Jersey, has logged 150 flu cases among students since Nov. 1, most of which were last week, Joe Cardona, the vice president of university relations, told ABC News. In response, the school has been holding flu vaccination clinics for students and staff this week.

Spikes in flu cases have also been seen at Florida State University and Florida A&M University, the Associated Press reported earlier this month.

Public health experts have warned that this flu season might be more severe, following last season’s mild flu activity.

Flu cases saw a significant drop during the 2020-2021 season according to the CDC — likely due in part to people wearing masks, practicing frequent hand hygiene and socially distancing to help limit the spread of COVID-19.

“As we head into respiratory virus season, it is important to take every mitigation measure we can to prevent outbreaks of the flu, RSV and COVID-19,” Dr. Natasha Bagdasarian, chief medical executive for the Michigan Department of Health and Human Services, said in a statement. “Wearing masks, washing hands, social distancing and getting vaccinated for the flu and COVID-19 will help prevent the spread of illness.”

ABC News’ Sasha Pezenik contributed to this report.