The Health Sciences Centre in Thunder Bay is dealing with an outbreak of Clostridium difficile, a bacteria that can cause severe diarrhea.
The manager of infection control at the hospital said there are five cases in surgical units on the third floor.
"We normally have it in the hospital in one or two cases, but we have it in an isolated area right now which causes us concern," said Hilary McIver.
"So we have put in place measures to try to prevent further spread. We have isolated these patients."
The patients are being treated and McIver said no one has died in the current outbreak of C. difficile.
The hospital's last outbreak was in 2010.
McIver said nursing and other staff are wearing protective personal equipment when they enter the rooms of the affected patients.
She added that the public is being asked to avoid visiting the hospital if they are ill, particularly with flu-like symptoms or any type of gastrointestinal symptoms. "We ask that you stay at home, so that you're not bringing it [into the hospital] to further complicate the situation," McIver said.