Reasons you might catch a virus: 5 & 6

Why am I always ill? | Coughs, colds and viruses | Always rundown

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5. You're overweight

Eating well and getting enough exercise get repeated all the time, but carrying too much weight can also cause you problems when it comes to getting ill.

'Your general lifestyle is clearly the biggest factor when it comes to getting coughs and colds,' says Dr Chris.

Results from a study at Boston University carried out on mice also discovered that obesity prevents the body from turning on its immune system properly, making it far more difficult to fight off infections.

In addition, too much sugar will also slow down your immune system. Sugar gets absorbed by white blood cells and prevent them from working properly - their normal job is to guard against infection - so allowing viruses to get into the body.

We suggest: Check your BMI with our calculator
Find out more: Get all the facts on obesity

6. You spend too much time inside

Sunlight is vital for our immune system, because it contains vitamin D, which is important not only for bones, teeth, but also a strong immune system.

'Our exposure to light is greatly reduced at this time of year,' says Kathleen McGrath. 'It's no coincidence that coughs and colds become less common as the days get longer.'

Many sufferers of SAD, the seasonal form of depression, show signs of a weakened immune system during the winter and are more vulnerable to infections and other illnesses. So even if it's not sunny, try to get outside.

We suggest: Take our quiz - How strong is your immune system?
Find out more: How to beat SAD