An 18-year-old man has been admitted to hospital with meningococcal disease in Whangarei - the region's fourth case this month.
In one of the other cases, a 1-year-old boy died in hospital.
The Northland District Health Board yesterday activated its incident management team in case the disease spread further - though the number of cases is in line with a typical year.
The region has had six cases this year.
The 18-year-old man was admitted to Whangarei Hospital's intensive care unit on Wednesday night.
Public health staff have identified close contacts of the young man to offer advice and preventive treatment.
Northland DHB medical officer of health Dr Clair Mills said the disease often led to complications.
"Unfortunately, yes. It's a very serious disease. It's invasive," she said.
A 1-year-old boy with the infection was admitted to hospital in a critical condition on August 9, and died after being transferred to the Starship hospital in Auckland.
"At this stage it's not an outbreak," Dr Mills said.
But the incident management team had been activated in case the hospital "started to see a whole lot more".