Rick Adams, executive director of the Queen Elizabeth Hospital, says six patients in Unit 3 were ill as of Thursday, down from 12 the day before.
He says one of the six patients fell ill on Thursday.
Patients reported symptoms including nausea, vomiting and diarrhea.
Adams says lab results have so far tested negative for common viruses known to cause such symptoms, but more test results are expected Friday.
In the meantime, he says the unit remains closed to visitors.