At least 342 people affiliated with the East Lansing school have tested positive for the coronavirus since Aug. 24, according to the Ingham County Health Department. In the three weeks prior, there were only 23 such cases, officials said. Cases started to rise once thousands of students returned to the area for the fall semester, officials said.
At least a third of the people who tested positive had recently attended parties or social gatherings -- and at least a third of those were associated with a fraternity or sorority, the health department said.
"This is an urgent situation," Ingham County Health Officer Linda S. Vail said in a statement issued Saturday. "The exponential growth of COVID-19 cases must stop. 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."
"If we do not slow the spread immediately, we will be dealing with the consequences across the county for months to come," she added.
Until 11:59 p.m. on Sept. 26, students are advised to only attend in-person classes, labs and athletic training. They can also leave for work and for food, medicine, medical care or other necessary supplies that aren't available via delivery.
The recommendation is not an emergency order, though the health department warned that "more stringent and mandatory restrictions will be imposed if students do not comply and break the transmission cycle."
The university stood behind the recommendation. In the statement, Michigan State physician David Weismantel said the quarantine "is another tool" to help them prevent the spread of the virus.
East Lansing Mayor Aaron Stephens also voiced support for the health department's recommendation. "While we know many students are doing the right thing, we are still seeing far too many social gatherings in the off-campus community, where individuals are in close contact without face coverings," he said in the statement.
Michigan State has limited in-person classes for the fall and has restricted the number of students living on campus. According to the Lansing State Journal, there are about 1,200 students living in on-campus apartments and under 2,000 in dorms. An indeterminate number of students live off campus in the area. The school has nearly 50,000 total students.
In July, all members of the university's football team had to quarantine or isolate after two members of the football staff and one student-athlete tested positive for COVID-19.
The University of Illinois is currently under a similar lockdown, as it attempts to quell a recent rise in cases. Through Sept. 16, undergraduate students are advised to only leave their residences for essential activities, such as getting food, working or taking mandated COVID-19 tests, and to avoid gathering in groups.
ABC News' Greg Bradbury and Matt Foster contributed to this report.