Influenza is not a straightforward illness, according to Dr Jim Miller, Medical Officer of Health at Toi Te Ora.
Influenza (the flu) is caused by three types of influenza virus – A, B and C – that infect the respiratory system. Symptoms include fever, chills, muscle aches, cough, congestion, runny nose, headaches and fatigue.
It is a common viral infection that is easily spread from person to person and can be deadly, especially in high-risk groups.
"It's quite a clever virus and good at spreading itself as, when our immune system responds to fight the virus, it can have changed itself a bit and cannot be recognised," Miller said.
"Even though it is less likely you will develop influenza with each changed strain, it can happen."
Every century or so a completely new strain pops up and, most often, results in a pandemic.
"When more people become unwell with influenza, there is more of a chance more people will become very sick from it."
Miller said underlying complications like asthma and respiratory and heart conditions could often be exacerbated by influenza.
"People can also develop complications with influenza that will result in a hospital stay.
"Other viruses like pneumonia may develop and a person already weakened with influenza may have difficulty fighting the virus, although less likely in a healthy person, it can happen to anyone."
The ESR Influenza Intelligence Weekly report stated flu and other respiratory virus activity was still unseasonably low in New Zealand, although activity was continuing to slowly increase.
National GP visits for influenza-like illnesses have increased in recent weeks.
Upper North Island DHBs are reporting the highest GP visit rates in those recent weeks. The national rate of Healthline calls for influenza-like illnesses increased again in the past week.
"The season is generally May to October so there is still time for an increase," Miller said.
"There is also still time to be immunised."