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.