Some surprising facts about colds, including what causes symptoms such as a blocked nose, and why mucus turns thick and yellow.
"It's your own immune response that makes you feel ill," says Professor Ron Eccles of the Common Cold Centre in Cardiff, where experts have researched the common cold for more than 20 years.
When you get a cold, the virus attacks the nose and the back of the throat, and it doesn't take long for the body's natural defences to start to work.
"The immune system detects the virus and floods the area with white blood cells and chemical messengers, and these trigger various symptoms such as headaches and a blocked nose."
"During a cold, the lining of your nose is the battlefront," says Professor Eccles. When your nose feels blocked, it isn't because it's full of mucus, but because the blood vessels in your nose are inflamed.
The nasal lining is made from erectile tissue (similar to the tissue in the sexual organs). When you have a cold, the blood vessels swell up as infection-fighting white blood cells flood to the area. This narrows the air passage in your nose and restricts the airflow as you breathe.
A decongestant spray can reduce the swelling and allow you to breathe more easily.
When an infected person coughs or sneezes, they release droplets of mucus into the air, or into their hand if they use their hand to cover their mouth. If you get these droplets on your hand (for example, by shaking hands or touching contaminated objects such as doorknobs), you can pass them into your eyes or nose when you touch them.
Most of us touch our eyes and nose more often than we realise. A duct links the eyes and the nasal cavity, and the virus travels easily from the eye to the nose and throat, where it can cause infection. You can help avoid being infected by washing your hands thoroughly.
Children get around seven to ten colds a year, compared with two to three for adults. So people who spend a lot of time with children, such as childminders, nursery teachers or school teachers, are also more likely to pick up the viruses.
When your immune system is fighting a cold virus, one of the first symptoms is clear, runny mucus from the nose. As the cold develops, mucus usually becomes thicker and yellow, then green. White blood cells cause this change in colour and texture as they flood to the nasal area and increase in number as the cold progresses.
"Many people think that yellow or green mucus is caused by bacteria, but this isn't the case," says Professor Eccles. "It's because there are billions of white blood cells in the mucus."
Important: Our website provides useful information but is not a substitute for medical advice. You should always seek the advice of your doctor when making decisions about your health.