BLOOMINGTON, Ind. — Indiana University says a suspected norovirus has sickened dozens of members of a fraternity.

Officials didn’t say which fraternity was affected, but students at the school in Bloomington who are ill were asked to not attend classes as they recover. University spokesman Chuck Carney says nearly all of the fraternity’s 52 members were ill and experiencing similar symptoms.

Carney says the illnesses started last week and continued this week.

Norovirus is sometimes referred to as the “stomach flu,” even though it is not a strain of influenza, which affects the upper respiratory system. It affects the intestinal tract and causes nausea, vomiting and diarrhea. Symptoms usually last 24-48 hours.