Royal Belfast Hospital for Sick Children deals with fungal infection
The Royal Belfast Hospital for Sick Children is dealing with an infection that can cause severe disease in children with weakened immune systems.
The BBC understands that one child acquired the infection while being treated for another condition.
Other potential cases are being investigated.
Aspergillosis is caused by a fungus or mould that can be found in heating or air conditioning systems.
In a statement, the Belfast Health Trust confirmed an increase in the level of the aspergillus fungus not only in the children's hospital but across the entire Royal group of hospitals site compared to previous years.
A spokesperson said that the "infection does present a risk to some groups of patients and that staff are working to reduce the risk posed by the environmental organism".
The condition mostly affects the lungs and can cause breathing difficulties and be chronic for asthma sufferers or those with cystic fibrosis.
It is understood the trust has been dealing with the issue for several weeks, but is having difficulty containing it.
Wards may have to be temporarily closed if other cases are confirmed.
The issue raises questions about the hospital's protection measures against the fungus and why the current system appears not to be fully working.