Seven wards in the 750-bed Morriston Hospital in Swansea have been shut to new patients because of a norovirus outbreak, the BBC reported Sunday.
Norovirus is a common viral infection easily passed from person to person that causes diarrhea and vomiting.
The hospital has been shut to most visitors, a spokeswoman for Abertawe Bro Morgannwg University Health Board said.
"The only exception is on the children's wards because that would be too distressing for the children not to have visitors," she said.
"There are also exceptional circumstances where a patient is at the end of their life. In that instance, we're asking people to ring us beforehand to arrange visiting."
The incubation period for norovirus can be anything from 4 hours to three days, and symptoms usually last 12 to 60 hours, the BBC reported.
"There's a lot of norovirus circulating in the community and people bring it into the hospital," the health board spokeswoman said.
"Visitors can also end up taking it out again. To try to break that cycle, we think it's better to close the hospital to visitors."