The school did not identify the athlete nor the football staff members who tested positive.
"Based on early results from COVID-19 testing conducted over the last week on football staff and student-athletes, which included a second staff member and one student-athlete testing positive on Thursday, all members of the football team will quarantine or isolate, while awaiting completion of a 14-day quarantine," MSU said in a statement.
"As part of the athletic department's return to campus policy, student-athletes quarantine when coming into close contact with an individual who tests positive for COVID-19."
The school did not disclose exactly how many athletes are now under quarantine, but an athletic department spokesman said it'd be a "majority" of the squad that had returned to campus.
A football team in the NCAA's top-flight football bowl subdivision includes 85 players on scholarship.
The quarantine abruptly stopped the Spartans preseason workouts and meetings, which could be up to 20 hours a week under NCAA rules, for a 2020 season that's in great doubt of even happening.
The Big Ten Conference announced earlier this month it would not play non-league contests this season, assuming there's a fall slate of games at all. The pandemic forced the Pacific-12 Conference to take similar action.
It also led to the shuttering of virtually all high-level sports around the world in mid-March, including the NCAA's wildly popular and profitable annual basketball tournament.