Llanidloes Graham Davies ward at Llanidloes hospital in Powys has closed to new admissions and to all visitors due to a possible diarrhoea or vomiting bug.
The ward will stay closed until no patients have any symptoms.
A spokesperson from Powys teaching Health Board said, "It is particularly important that people who are unwell keep away from patients in hospitals and long-term care facilities as they may be at increased risk of contracting gastroenteritis.
"If they do contract it they may experience more severe symptoms and it may exacerbate their other health problems."
"It is important that, if patients being admitted to hospital, or their household contacts, have been suffering from gastroenteritis within the last 48 hours, they should inform the admitting doctor."
More top news
More needs to be done to close the gap between education attainment and poverty, says an Assembly Committee.
The parents of a baby born prematurely say their son's life was put in serious danger after being given out-of-date donated breast milk.
The UK Government is to announce a range of new powers for the Welsh Assembly in what it calls a 'landmark moment' in devolution