A baby died of the disease on October 8, within 48 hours of becoming unwell - the only fatality from the disease so far this year.
Meningococcal is a bacterial infection that can cause two serious illnesses: Meningitis - an infection of the membranes that cover the brain; and septicaemia - blood poisoning.
Dr Mills said meningococcal often manifested like a viral flu or mild illness at the start.
"But it can be rapidly life threatening, especially in babies and young children," she said. "If your baby is unwell, please see a doctor and watch out for danger signs such as fever, vomiting, refusing to breast feed and fast breathing."
Older children and adults could experience headaches, fever and vomiting.
"The tell-tale purplish rash may not present until very late in the illness so don't wait if you are concerned," Dr Mills said.
Other symptoms could include sleepiness, joint and muscle pains, a stiff neck, aversion to bright light or a reddish-purple "pin prick" rash.
Dr Mills advised anyone with such symptoms to contact their doctor immediately or call Healthline free on 0800 611 116.
If patients had seen a doctor and gone home, but were still concerned, they could call their doctor again.