Visitors and new patients are to be stopped going onto a hospital ward after an outbreak of the winter vomiting bug, norovirus.
Ward C at Morriston Hospital in Swansea has been closed to prevent the spread of the bug.
The surgical ward has six patients with norovirus symptoms.
If a significant number of wards have to be closed because of the bug, the whole hospital will be closed to visitors, managers said.
Director of Nursing Victoria Franklin said: "This winter norovirus has started to peak sooner than usual, and we have already had to close some wards.
"Our hospitals are also under a huge amount of pressure because of the unusually cold weather, which has increased the number of emergency patients being admitted.
"Closing wards because of norovirus is a necessary action, but it does mean that beds are taken out of commission while the ward is closed. This can cause delays in admitting patients.
"This is another reason why we need to take additional measures to deal with norovirus as quickly as possible."
Earlier this month, 157 people were affected by an outbreak at the Park Plaza Hotel in Cardiff.