Health authorities suspect measles may now be widespread in Northland and they're hearing only about the most serious cases.
Nine cases have been confirmed in the region, three of them since last week and one of those people is in intensive care with complications.
Medical Officer of Health Catherine Jackson said the new cases had had no contact with the earlier ones, and that was worrying.
"We don't know where these people have picked up their measles from, despite taking a really detailed history.
"They're not linked to any of our previous cases so they could have picked it up anywhere in the community."
What was concerning was that while they were infectious, some of those people had been visiting sick relatives in hospital, she said.
"It's really important not to visit people in hospital when you're unwell. Even if the patients have been immunised, they are at higher risk of catching measles (because they are sick)."
Dr Jackson says people with a mild case of the measles, or in the early stages of infection, might not even realise they had the virus.
"The measles starts off looking very like the flu and because the flu season has started it is quite difficult for people to know. But...with any infection, it's best to stay at home and away from other people so you stop passing illnesses on. "
Measles was ten times more infectious than the flu, Dr Jackson said.
A third of the Northland cases have been admitted to hospital, and nationally the figure is close to 50%, well above the admission rate in previous measles outbreaks.
Dr Jackson said that suggested the virus was more widespread than previously thought.
"We normally expect ten to twenty percent of people to be admitted so what this really tells us is that there's more out there than we know about. It can be mild or severe and we think there are milder cases in the community that aren't sick enough to be coming to the attention of a doctor."
But those people were just as capable of spreading measles as people who were more seriously ill, she said.
30 people are now in quarantine in Whangarei as a result of the latest cases, who range in age for 25 to 58.
Most people over 50 were immune but a few people in that age group had not had measles as children and were vulnerable, Dr Jackson said.
"The group we're most concerned about is our young adults - the 13 to 30 year olds. We think about one in four have not been immunised."
Anyone uncertain about whether they'd been vaccinated as a child could get free vaccinations from their GP, Dr Jackson said.
"There is no risk in having the extra shots even if you were immunised - and it's the best way to avoid what can be a very serious illness."