Health bosses are working to establish whether three cases of tuberculosis at Belfast City Hospital are linked.
Screening is being offered to 43 former patients of Ward 6 at the hospital who may be at risk of developing tuberculosis (TB) after it emerged three people have been diagnosed with the potentially deadly infection.
The first case - an elderly patient who was treated in Ward 6 - was diagnosed at the end of January.
A spokesman from the Belfast Health & Social Care Trust said the patient has since died but stressed he did not die from TB.
He said a nurse who treated the patient was subsequently diagnosed with TB but she is now receiving treatment and is recovering from the bacterial infection.
Dr Tony Stevens, medical director of the Trust, said: "We have established the first two cases, between the patient and staff member, are linked.
"This is extremely unusual, in my 30 years working in the health service I have never seen anything like this.
"We have already carried out screening of people we believe would have been in close contact with the first case and as a result we have ascertained a third person has TB.
"Tests are now being carried out to determine whether the third case is linked but it will be two or three weeks before we have the results."
Dr Stevens said the Trust will carry out a full investigation into the outbreak.
"I think this serves as a reminder that TB has not gone away, we see about 50 to 60 cases annually in Northern Ireland and doctors and nurses should be mindful of this," added Dr Stevens.
TB is a bacterial infection spread through inhaling tiny droplets of saliva from the coughs or sneezes of an infected person.
It mainly affects the lungs but can spread to many parts of the body, including the bones and nervous system. Typical symptoms of TB include a persistent cough, weight loss and night sweats.
The Belfast Trust has said the screening is a precautionary measure in line with standard guidance and patients should not be alarmed and that anyone considered to be at risk will have received a letter by yesterday.
With treatment, a TB infection can usually be cured.
Most people will need to take a long-term course of antibiotics.
Before antibiotics were introduced, TB was a major health problem in the UK.
Nowadays, the condition is much less common but in the last 20 years TB cases have gradually increased, particularly among ethnic minority communities originally from places where TB is widespread.
In 2009, 9,040 cases of TB were reported in the UK. Most of these occurred in urban centres, with over one-third of cases in London.
Globally, TB remains a major public health problem. There were 9.4 million new cases of TB in 2009, and 1.7 million deaths from the condition.
Countries with high numbers of HIV cases also often have high numbers of TB cases.
The Trust has set up an advice line for anyone who is concerned which is available between 9am to 5pm until Friday.
The telephone number is 0800 9178 226.