(CNN)Health officials on Saturday asked Michigan State University students to self-quarantine immediately after parties contributed to 342 new coronavirus cases.
"At least a third of new cases recently attended parties or social gatherings, and at least one third of those gatherings are associated with a fraternity or sorority," the Ingham County Health Department said in a statement.
The hundreds of new Covid-19 cases involving people affiliated to the university date to August 24, the health department said. Only 23 people had tested positive before that date.
"This is an urgent situation," said Ingham County Health Officer Linda S. Vail. "The exponential growth of COVID-19 cases must stop."
The outbreak started as students returned to the East Lansing area for the fall semester, the department said. Most MSU classes are online but "many students had binding off-campus leases or simply desired to physically return to the university community."
"MSU is committed to doing everything we can to prevent the spread of COVID-19," Michigan State University Physician David Weismantel said in the statement. "The safety of our entire community is a priority and we all have a role to play in preventing the spread of the virus. This recommendation from the health department is another tool to help us do just that."
The quarantine recommendation isn't an "emergency order," but the health department warned that "more stringent and mandatory restrictions will be imposed if students do not comply and break the transmission cycle."
The health department said it will look at houses licensed for more than 10 unrelated people "to see if additional measures are warranted."
"I am concerned about the health and safety of the MSU community and importantly, I am seriously concerned that unchecked transmission locally will affect the health and safety of all Ingham County residents," Vail said.
East Lansing Mayor Aaron Stephens, in a statement, urged students to consider their role in halting the "community spread and, ultimately, saving lives."
He added: "We are still seeing far too many social gatherings in the off-campus community, where individuals are in close contact without face coverings."
Students are asked to self-quarantine for 14 days, until 11:59 p.m. on Saturday, September 26. They are permitted to attend "in-person instruction, labs, and intercollegiate athletic training," the health department said.
"They may also leave their homes to work or to obtain food, medicine, medical care, or supplies that are needed to sustain or protect life when such cannot be obtained via delivery," the health department added.
The entire MSU football team was placed under 14-day quarantine on July 23 after a second staff member and student-athlete tested positive for Covid-19.