VIRUSES are simple organisms about a hundred times smaller than human cells – but they are incredibly effective at infecting people.

Vaccination works by tricking the body into thinking it has been infected by a virus, in order to make it develop resistance through recognising the proteins on the surface of the virus.

But the constant mutation of these proteins means experts have to predict what strains will be circulating every year and develop fresh vaccines to respond.

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Symptoms of flu include fever, headaches, extreme tiredness, cough, sore throat, nasal congestion, and body aches.

Even fit and healthy people can feel the effects for up to two weeks, and serious complications can include pneumonia, bronchitis, meningitis and encephalitis (inflammation of the brain). It can also lead to the worsening of chronic conditions, such as congestive heart failure, asthma or diabetes.

The key groups who are advised to get the flu jab include people aged 65 and over, pregnant women, and under-65s with existing medical conditions, such as asthma, diabetes, cystic fibrosis, multiple sclerosis and other heart, lung and liver diseases. The vaccine contains an inactive version of the virus which cannot replicate and cause illness.

Infections expert Hugh Pennington, emeritus professor of bacteriology at Aberdeen University, said the swine flu outbreak in 2009 demonstrated just how unpredictable the flu virus can be.

He said: "The surprise with that was where it started: it wasn't predicted to start in Mexico, it was predicted that if it was going to start anywhere it would be in somewhere like China, as that is where the previous pandemics seem to have originated from.

"It started at the wrong time of the year and then it was much milder than people thought it might be. There were lots of surprises and that is just the pattern with flu – what it does is take you by surprise."