A member of staff at Derry's Altnagevin hospital has been diagnosed with tuberculosis.
Authorities at the hospital have written to some 1,400 patients who may have come in contact with the casualty department worker.
Their GPs have also been notified in a bid to make them aware of the symptoms and telling them how to deal with any concerns.
Even though warnings have been sent out, hospital chiefs stressed Northern Ireland has one of the lowest infection rates in the world.
Dr Richard Smithson, a consultant in communicable diseases at the Western Health and Social Services Board, said there was almost no risk topatients.
He told BBC Radio Ulster: "We have gone through all the records and identified every patient who would have had contact with this member of staff.
"Letters have been sent out to them and should be arriving with patients this morning. Anybody who doesn't get a letter can be reassured they weren't in contactwith this member of staff".
PA