The Eastern Health Board has warned parents of children and young adults to be vigilant for signs of meningitis, which is more prevalent during winter.
Dr Lelia Thornton, an EHB public health specialist, said that parents should familiarise themselves with the symptoms of the disease.
Meningococcal meningitis, she said, may initially present as a flulike illness, with symptoms such as headache, vomiting, fever, widespread aches and pains and sore throat, making initial diagnosis difficult.
However, it could then progress to symptoms such as sensitivity to bright lights, neck stiffness, confusion or drowsiness.
She said that another important sign to watch out for was rash. "This may start as just one or two tiny pinpoint spots or blotches anywhere on the body. The spots may turn into tiny bruises which don't turn white when touched."
In babies, the signs are non-specific, but include fever, being off feeds, irritability, high-pitched crying, vomiting and drowsiness.
She pointed out that meningitis was not confined solely to young children, emphasising that cases could occur at all ages.
Two teenage sisters from Co Antrim were admitted to hospital in on Monday suffering from suspected meningitis.
The Northern Health Board said last night that one of the girls was being treated in intensive care at the Antrim Area Hospital. However, both were reported to be in a stable condition.
Members of the girls' immediate family were treated with antibiotics following their admission to hospital and information has been distributed to their school, St Mary's in Magherafelt.