A ward at the Royal Glamorgan Hospital has closed due to the winter vomiting bug norovirus.
Cwm Taf Morgannwg University Health Board has confirmed that ward 19, an acute medical (respiratory) ward, was shut on Monday to new patients and visitors to stop the spread of the virus.
A spokeswoman for the health board is urging people not to visit the Llantrisant site if they have experienced diarrhoea or vomiting symptoms in the last 48 hours.
Norovirus is the most common stomach bug in the UK and affects between 600,000 and a million people in the UK every year.
The first sign is usually a sudden sick feeling followed by forceful vomiting and watery diarrhoea.
Some people will also have a raised temperature (over 38°C/100.4F), headaches, painful stomach cramps, and aching limbs.
There is no cure for it and it normally lasts for no more than a couple of days. However it can prove particularly damaging to vulnerable hospital patients and those in care settings.
NHS chiefs are urging people to wash their hands regularly with soap and water, rather than using hand sanitisers which do not kill the virus.
Norovirus has led to wards closing in several hospitals previously over Wales this winter, including the University Hospital of Wales in Cardiff, Morriston Hospital in Swansea, Prince Charles Hospital in Merthyr Tydfil and Princess of Wales Hospital in Bridgend.
Several schools also had to shut to protect pupils and staff.
