THIS is a golden age for the common cold. Densely populated cities provide ideal breeding grounds for the viruses that readily jump from one continent to another by jet.
The average adult gets two to five colds a year. School children suffer the worst, with seven to 10 colds a year. Young children with undeveloped immune systems are the most vulnerable and form the main reservoir of infection. Most colds are transmitted at home or school and adults in regular contact with children are most at risk of infection.
As I get older, I take more care to protect myself. I wash my hands a lot and I try not to touch my nose and eyes in public. Your fingers can easily become contaminated with viruses by touching door handles and by shaking hands. You may then touch your nose and eyes and infect yourself. Tears from the eye drain into the nasal cavity, passing viruses into the nose.
Hand-washing can reduce the spread of colds within the family. Kissing is OK, though. The virus travels in the respiratory system, so is lodged at the back of the throat. Only if you have a bad cough and some of the respiratory mucus becomes mixed with your saliva is there a risk of the virus being transmitted in a kiss. Cold viruses appear to be spread by large particles expelled at close range by coughs and sneezes. The only guaranteed way to avoid a cold is to become a hermit.
Although few of us escape at least a couple of colds each year, common cold viruses are not very contagious. Under laboratory conditions with healthy volunteers, it has proved remarkably difficult to spread infection from one person to another. You are most infective when you have the early symptoms of sneezing, runny nose and cough. The virus replicates in the cells lining the nose and are coughed or sneezed out in droplets of mucus. The virus is also spread on fingers when we contaminate them with secretions from the nose.
There may be genetic differences in our immune systems that make some people more vulnerable. Most viral infections produce no disease -- this is the iceberg concept of infection. They do not cause symptoms and pass without notice. Or they may cause minor symptoms such as a throat irritation and a couple of sneezes which are not recognised as a cold.
The "stinking colds" that cause severe symptoms represent the tip of the iceberg of infection. Genetic differences in the immune system may explain why the same virus causes severe symptoms in one person and mild or no symptoms in another.
I don't think there is any evidence that men respond more strongly to infection than women. But women's immune systems are different from men's because they have to support a foreign body when pregnant. So it may be that their immune systems are more tolerant.
This could be important in a flu pandemic because it could affect its impact. It is obvious that if there are sex differences there could be differences affected by genetic make-up, too.
The pressure of everyday life can make you more susceptible to infection. Experiments on volunteers show they are more likely to become infected if they have recently suffered problems at work or home. Stress is associated with the suppression of general resistance to infection.
It is not clear how it affects the immune system but there appears to be an increase in corticosteroid hormones, which are known to decrease resistance. The increased stress of modern city life may be one of the factors behind the very high incidence of common cold infections in our cities.
As soon as the weather turns cold, I try to keep my nose warm. As winter comes, we put on our overcoats to protect our body temperature but we don't protect our nasal temperature. The nose dries out in low temperatures or in dry air-conditioning, and respiratory viruses become trapped and start to reproduce.
One cold study showed that when volunteers were chilled by sitting with their feet in icy water for 20 minutes, they were three times more likely to develop a cold in the next five days. We should all have scarves round our necks and over our noses. I must admit, I am not good at following my own advice because you tend to look like a bandit with a scarf wound round your face. It would be more acceptable in Canada.
This old saying is part of folklore. Its origin lies with the fact that any feverish illness is likely to be associated with loss of appetite. Most colds in adults are not associated with fever and so there is not much loss of appetite.
It is the release of chemicals from the white blood cells called cytokines that fight infection and cause fever, tiredness and loss of appetite. In general, common cold viruses do not trigger a large response from the white cells but severe cold infections can, like flu, cause a lot of cytokine release and give us the typical flu-like symptoms of fever and tiredness and loss of appetite.
When I was a PhD student in the 70s, I did some work on bradykinin: I demonstrated its presence in nasal secretions from people with colds. The symptoms of a cold -- sneezing, coughing, runny or blocked nose -- are caused by a complex soup of chemicals triggered by the immune system, which act on blood vessels, glands and nervous tissue. They are part of the inflammatory response to infection.
Bradykinin, a peptide, is one of these chemicals and has been proposed to play a dominant role in colds. As well as being identified in nasal secretions, if put into the nose in healthy volunteers it causes nasal congestion and irritation.
There is a lot of interest in developing a medication that would act as a bradykinin antagonist. It would work in the same way as antihistamine against hay fever, reducing the symptoms. But no one has come up with one and it is a long time since bradykinin was discovered.
If such a development is to take place, it is likely to come from arthritis research as bradykinin is also implicated in the pain and swelling associated with joint inflammation.
There is no good evidence that either can prevent a cold. The Nobel Prize winner Linus Pauling popularised the idea of taking daily doses of vitamin C to ward off colds and sales of the vitamin soared.
However, only a very small proportion of the population in the West are likely to have any deficiency of vitamin C and it is unlikely a daily dose will provide any benefit.
Echinacea is the most popular preventive treatment for colds and helps to prevent infection by boosting the immune system. It could, in theory, abort a common cold infection and prevent the development of symptoms, but again there is no evidence of this.
Only the immune system can cure a cold. In most cases, it will take four to seven days and will not require any other treatment.
If you want relief from severe symptoms, aspirin, paracetamol and ibuprofen are very effective for relieving headache, sinus pain, sore throat and aches and pains.
A nasal spray containing xylometazoline or oxyetazoline to clear a blocked nose can be very helpful before sleep.
Any hot drink will relieve a cough and sore throat -- but standard cough medicines are no better than placebo. Spicy foods and hot soups are also beneficial.
Professor Ron Eccles is director of the Common Cold Research Centre at Cardiff University. He was interviewed by Jeremy Laurance