The common cold: Causes and symptoms

Professor Ron Eccles sniffs out the causes and symptoms of the common cold
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The Independent Online

What is a common cold?

Common colds are caused by viruses that infect the nose and throat. They have a familiar set of symptoms that we all recognise: sneezing, runny nose, sore throat, blocked nose and feeling tired. Other symptoms may include headache, loss of appetite and a feeling of chilliness or fever.

Adults suffer from two to five colds in a year, while children suffer from seven to 10. All animals are infected by viruses and catch colds. You may see your cat or dog with a cold and even seals and camels suffer. The viruses are specific for each animal so they rarely jump from one species to another, which thankfully means you can't catch a cold from your pet!

How do we catch a cold?

You catch a cold when a common cold virus gets into your nose or throat. The virus must come from someone else with a cold and travels from one nose to another in droplets of mucus (otherwise known as snot!) sprayed out when we cough or sneeze. Viruses also travel on dirty fingers, contaminating surfaces such as door handles and computer keyboards.

The only sure way to avoid getting a cold is to become a hermit. You cannot catch a cold by kissing as the virus is found in snot and not saliva. Colds are more common in cold weather and this may be because our noses are colder; the cooling slows down the conveyor belt of mucus in the nose that normally traps viruses.

How big is a common cold virus?

A common cold virus is 20 nanometres in diameter - there are one million nanometres in a millimetre, which means you could place 50,000 viruses end to end along a line only one millimetre long. If the common cold virus was as large as an apple, your nose would be the size of Wales and your body the size of Europe!

Why is it difficult to find a cure?

There are over 200 viruses that cause colds in humans, and with so many viruses it is too expensive to develop a vaccine. Another problem is that the viruses are constantly mutating. A chemical cousin of DNA called RNA forms the genetic code of many common cold viruses. However, unlike DNA, it does not correct mistakes in its make-up. So, when viruses replicate in the cells of our nose, errors may occur in the RNA code of said viruses which sometimes leads to a new and even more successful strain of virus. "Success" in this case means causing more colds and spreading right around the human population of the world.

Eliminating smallpox was much easier than trying to eliminate the common cold. Smallpox was caused by a single stable virus, and therefore it was possible to develop a vaccine and eliminate it. A similar job is now underway to eliminate polio.

What are the latest ideas in finding a cure?

Unlike the viruses that give us a tummy upset, the common cold viruses are destroyed in the acid environment of the stomach; you won't catch a cold if a colds virus is put in your mouth because the virus is swallowed and destroyed in your stomach. A new way of defeating the colds virus is to use a slightly acidic spray in the nose at the very first symptoms of a cold. The mild nasal spray does not irritate the nose too much but it does inactivate the colds virus, and may be the first step in developing a cure.

Putting it all in perspective

Viruses are the most successful form of life; every animal, plant, fungus and bacteria has them. As long as there are humans there will be viruses that get up our nose and cause colds. Even if we were to destroy every human virus it is likely that we would eventually pick up new ones to replace them from those that infect our domestic animals. So, thank goodness we have an army of white cells in our blood that fights any virus that enters our body. All hail the immune system!

Professor Ron Eccles is director of the Common Cold Centre, Cardiff University,


NHS Direct

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Molecular biology

Further information on DNA and RNA