A national outbreak of whooping cough has been declared by the Ministry of Health.
There had been a total of 1315 cases since the start of this year, it said.
Director of public health Caroline McElnay said babies under one year old were most vulnerable.
Dr McElnay said the best way to protect against whooping cough was for babies to get free immunisations when they were six weeks old, three months old, and five months old.
Pregnant women should get vaccinated between 28 and 38 weeks of pregnancy to protect the child until they're old enough to be immunised.
Outbreaks of the disease happen every three to five years - the most recent spanned August 2011 to December 2013.
During the outbreak hundreds of babies and children needed to go to hospital, and three died.
Health professionals are expecting the outbreak to turn into an epidemic.
Reported cases of whooping cough have spiked in the past four weeks.
Immunisation Advisory Centre director Nicky Turner said pregnant women should get immunised. She said the mother passes the protection on, protecting the baby for the first few months of their life.
She said it was very important babies were vaccinated on time.
Immunisation is free for mothers and babies.