50% of flu cases don't have any symptoms so people, particularly those in at-risk groups, may carry the virus and pass it on, without knowing it.
This is a risk to everyone who gets the virus but also to their friends, family and relatives, who risk catching flu too.
Last winter we saw the significant effect that flu can have on people's health, and in turn how it stretches the capacity NHS staff and services.
During the peak of the outbreak the proportion of GP appointments and 111 calls related to flu were around double the previous year, and more than 1 in 8 of all A&E consultations was for acute respiratory infections.
The single best way of avoiding the flu is to get vaccinated, and for 22 million of us in England that vaccination is free on the NHS through our GP or pharmacist.
This year, people aged 65 and over are being given a new type of vaccination which is the most effective for this age group available anywhere in the world.
Getting this vaccination and avoiding flu will save hundreds of lives and thousands of trips to the GP or hospital.
There are more than enough doses of this new vaccine available to meet expected demand, but unlike in previous years, the sole supplier has phased deliveries to cope with global demand.
While this may have meant some people weren't able to get vaccinated on their first attempt, all vaccines now have been delivered to those surgeries and pharmacists who placed an order on time, which means that all eligible people can get their jab before the flu season really starts to kick in later this month.
If you haven't yet had your jab, now is the time to go get protected.
To find out if you or your loved ones are among the 22 million people in England eligible for a free NHS flu vaccination, and for other tips on staying well this winter, head to www.nhs.uk/staywell.
Written by Jane Cummings, Chief Nursing Officer for England and Executive Director at NHS England