The official flu "season" starts in early November and goes well into the following year in the UK.
The culprit: A flu virus
Even in a normal year, millions will fall ill - and some will die as a result of complications.
But what is flu, and what can be done to prevent it and treat it?
What is flu?
Flu, short for influenza, is correctly described as a respiratory illness - it affects the upper or lower part of our lungs and windpipe.
Of course, as well as a severe cough, there are other well-known symptoms.
These include fever, shivering, headache, and muscle aches.
Many people confuse flu with a heavy cold, but flu is more serious. Not only does a cold generally not last as long, but the symptoms, though unpleasant, are more tolerable, and the chance of serious complications less.
Can it be serious?
For most people, it's just a highly unpleasant way to spend between a few days and a week.
However, it can lead to complications, which, in the vulnerable, could be life-threatening.
The most common of these is pneumonia, but even bronchitis caused by flu can be a problem.
Those most likely to need hospital treatment are the old and the very young, and those with chronic illnesses that leave them weakened, such as heart disease, or more vulnerable to chest problems, such as asthma or COPD.
What causes flu?
There are a wide variety of influenza viruses, which can be broadly split into a few different types.
This year, a strain of influenza "A" called Fujian-like appears to be dominant in the UK. It appears to be most virulent in younger children.
In addition, other viruses such as rhinoviruses, respiratory syncytial viruses and adenoviruses can cause similar symptoms.
How is it spread?
Flu is very good at spreading. It can be carried in droplets sprayed out when an infected person coughs or sneezes.
When it hits cells in an uninfected person's breathing passages, it can start to replicate within six hours.
From the onset of symptoms, a patient is infectious from between three and five days in adults, slightly longer in children.
There is a theory that it is rampant in temperate countries in winter because people stay indoors, hastening its spread.
Are there any drugs to treat it?
Drugs are beginning to emerge which can tackle the flu viruses.
Antivirals such as Relenza and Tamiflu may, if taken within a couple of days of the onset of symptoms, shorten the flu attack by between one and 2.5 days.
Can I stop myself catching it?
Yes. Because new flu strains tend to emerge in the far East months before arriving in the UK, it is possible for experts to predict which ones are coming here, and develop a vaccine against them.
The vaccine contains inactivated copies of a few of the most likely viral strains, and offer a relatively wide protection against any similar strains that infect people here.
The viruses are grown in hen's eggs before being killed and put into a vaccine.
When the immune system encounters these inactivated viruses, it still makes defences against them - so when the real thing turns up later, it is far more ready to destroy it.
In the UK, vaccination is recommended - and free - for persons aged over 65, as well as those with immune compromising illnesses such as HIV, diabetics, and those with chronic heart or lung problems.
Young healthy adults do not get the vaccination free on the NHS, as they are not particularly threatened by the illness.
Because the strains of flu change every year, a new jab is needed before each flu season.