Flu is a catch-all term used to describe a variety of illnesses ranging from a blocked nose to aches and pains.
Although viruses cause both, flu is not just another name for a bad cold.
The true version (influenza A or B) is different from the common cold, and can be a killer. Although the infection is limited to the breathing passages, the symptoms affect the entire body and can be very debilitating.
One symptom of real flu is that you feel so bad you simply do not have the energy to get out of bed. You also have aches and pains all over your body, perhaps a headache and maybe a cough, often a hacking one.
A fever or chills may develop. Other symptoms are sweating, a sore throat and fatigue. A cold's symptoms more typically include sneezing, runny or blocked nose or dry and itchy throat.
How did I get it?
Flu is a respiratory virus picked up from somebody coughing or sneezing near you. It moves down the respiratory tract and most healthy people will fight it off, taking paracetamol for the aches and pains. They recover completely, usually within 10 days, from an attack.
But vulnerable people are at more risk. The influenza pandemic of 1918 killed more than 20 million people worldwide, more than the number killed in the first World War. Those at greatest risk are the very young and the old, those with existing illnesses such as asthma, diabetes, heart disease and chronic bronchitis and those with inefficient immune systems. Serious complications may develop including pneumonia. Most deaths from influenza occur among the elderly.
What about vaccines?
The Office for Health Gain began an influenza vaccination campaign in September, aiming at people over 65 years, those who have lung, heart or kidney disease, diabetes or a suppressed immune system. Research shows annual vaccination of these people can reduce infection.
But vaccines are not always effective. There are many strains of flu circulating. Each year experts from the World Health Organisation identify the strains that will strike in order to prepare the appropriate vaccine. But they do not always get it right.
While antibiotics may help treat the complications of influenza they do not work against the virus that causes it.
I have the flu, what should I do?
Get plenty of rest
Keep warm
Drink plenty of fluids
Eat what you can
Breathe in steam from hot (not boiling) water.
See the doctor if it is not improving within a few days.
There is no cure for the flu, although pharmaceutical companies are competing to develop a new type of drug - called neuraminadase - to stop the virus from spreading within the body.