Northland District Health Board medical officer for health Dr Catherine Jackson said flu signs and symptoms could be similar to the beginning of measles so those who were unwell should not visit family and friends in hospitals until they recovered.
She is also advising pregnant women to be immunised against influenza and that they were a priority group when vaccine supplies ran low.
A recent New Zealand study showed that pregnant women were nearly five times more likely to be hospitalised with influenza than non-pregnant women, Dr Jackson said.
Influenza immunisation can be given at any time during pregnancy and is free from any GP.
"Influenza vaccine in pregnancy protects you, reduces the risk of stillbirth, and protects your baby from influenza during the first few months of their life."
The latest data on influenza from the Northland DHB goes back to April.
It shows one in four or 100 from the 400 patients tested positive during that month at all Northland DHB-run hospitals in the region.
Dr Jackson said whooping cough - or pertusis - is another illness to watch out for, especially among newborn babies and young infants who have the highest risk.
There are 39 confirmed and two probable cases of whooping cough in Northland.
"For every 100 babies who get whooping cough, around 70 will be hospitalised, seven will need intensive care, and one will have long-term complications.
"There is no treatment for whooping cough in young babies so occasionally babies in New Zealand die from whooping cough."
She said immunisation during pregnancy was the best way to protect newborns because the antibodies a mother makes to the vaccine crossed the placenta and were 92 per cent effective in protecting the babies until they were old enough to be immunised.
Immunisation in pregnancy is free from any GP from 28 weeks of pregnancy.
Dr Jackson said now was also a good time to remind people of precautionary measures to take against the spread of measles— another highly infectious viral illness which has similar signs and symptoms to influenza.
Meanwhile, there have been no new cases of measles in Northland, with the total standing at nine. Three patients aged between 25 and 58 confirmed to have measles lately were all unimmunised and living in Whangārei.
Influenza is a viral infection that attacks your respiratory system — your nose, throat and lungs. Influenza is commonly called the flu, but it's not the same as stomach "flu" viruses that cause diarrhoea and vomiting.
For most people, influenza resolves on its own. But sometimes, influenza and its complications can be deadly. People at higher risk of developing flu complications include:
Young children under age 5, and especially those under 2 years
Adults older than age 65
Residents of nursing homes and other long-term care facilities
Pregnant women and women up to two weeks postpartum
People with weakened immune systems
People who have chronic illnesses, such as asthma, heart disease, kidney disease, liver disease and diabetes
People who are very obese, with a body mass index (BMI) of 40 or higher
Tips to avoid catching influenza:
* Get a flu jab
* Avoid close contact with those who are sick or other people if you are sick
* Stay home from school or work for at least 24 hours after your fever has disappeared
* Use a tissue to cover your nose and mouth when sneezing and coughing.
* Regularly wash your hands with soap and water or an alcohol-based hand rub.
