Morriston Hospital in Swansea is closed to visitors and seven wards are shut to new patients because of a norovirus outbreak.
A spokesperson said visitors would only be accepted for children and in exceptional circumstances.
A ward was closed last month and managers warned the hospital would shut to visitors if more wards were affected.
There are about 750 beds at the hospital.
A spokesperson for Abertawe Bro Morgannwg University Health Board said seven wards were shut due to norovirus so it decided to close the hospital to visitors.
“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.”
“There’s a lot of norovirus circulating in the community and people bring it into the hospital”
Hospital spokesperson
The hospital said the no-visiting decision applied not only to the seven wards, but all wards.
However, this does not affect the accident and emergency department.
The spokeswoman added: “There’s a lot of norovirus circulating in the community and people bring it into the hospital.
“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.”
Ward C, a surgical ward at Morriston, was closed last month to prevent the spread of the bug after six patients had norovirus symptoms.
Hospital managers said at the time the spread of the virus could be controlled even more by stopping general visiting.
It was suggested the whole hospital could be shut to visitors if a “significant number” of wards were closed through norovirus.
Norovirus is a common viral infection which causes diarrhoea and vomiting, and is easily passed from person to person.
The incubation period is anything from four hours to three days, and symptoms usually last 12 to 60 hours.
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