We all have our pet theories about the weather and illness. One widely-held one is that a mild winter leads to a lot more flu and colds because 'the viruses haven't been killed off by a cold snap'.

In fact there hasn't been any evidence to support it - until now. There's no correlation here between winter weather conditions and flu epidemics. And despite their name, colds are just as common in the summer months as in the winter. We don't know for sure why they are called colds. It is more likely to be after the chill we often feel at the start of the illness (which is actually a sign of a high body temperature) than the outside temperature.

So until now, the scientists have pooh-poohed any thought that our weather makes a difference to any illness. That may well have to change. Because there is one part of the world where the weather has changed enormously in the last few years - and there has been a big increase in infectious diseases along with it.

It is Peru. The El Nino change in the ocean currents there has brought with it a five degree rise in the average temperature. That has led to three times as many children as usual having viral infections - mainly causing diarrhoea. This may not sound relevant to us, but one Peruvian children's hospital has had to admit more than 57,000 under-10 year olds with viral diarrhoea to save their lives during the El Nino period. This is a colossal increase in the demands on their services.

The doctors point out that these children were not all from poor districts without normal health services where infections are to be expected. Many came from good homes and had parents with good understanding of hygiene. They predict that other countries where global warming is pushing up winter temperatures may suffer similar problems. As we get warmer, the germs that infect us seem to spread more easily - and the type of illness may differ from country to country.

The process may have started here already. This winter has been the warmest and wettest most of us can remember. And we doctors have been swamped by outbreaks of viral illnesses that seem to have hit more people than usual. It isn't just the flu we had during January - that wasn't a particularly big epidemic. GP colleagues have told me that they can't remember a year starting with so many different viruses around. There were viruses that caused flu-like symptoms but weren't flu. There seemed to be a lot more cold viruses around. And lately there has been a virus that causes people to feel nauseated for a week or so - hitting adults and children alike.

This was an odd one, because it came with constipation, rather than diarrhoea. It hasn't been identified yet, but we suspect it's caused by a member of the group of 'enteroviruses' to which polio belongs. However, it isn't severe, and everyone with it appears to have shaken it off eventually.

It's important, if you have it, to drink as much watery fluid as you can, without making yourself sick. A few sips every 20 minutes or so is better than drinking a lot every few hours. And rest until you no longer feel sick. Get as much sleep as possible. Most important of all, don't go back to work, or send the kids back to school too soon. Wait a couple of days after your last sick bout to build up your strength again - chicken soup is a brilliant restorer - before returning to the stresses and strains of normal life.

There are other implications for us in this news from Peru. It's not only changes in temperature that seem to make us ill - but we now suspect that changes in pressure make a difference, too. El Nino affects us, too, in that we are experiencing far more changes in pressures than before. Low pressure zones follow one another with monotonous regularity, with only short periods of high pressure in between.

I've suspected for some time that many people with joint and nerve troubles do far worse during days when the pressure is dropping, and improve when it rises again. This may not be just a case of preferring sunshine to rain, but because the body's tissues may not adapt to the changes in outside pressure fast enough.

So joints and susceptible nerves may swell, giving joint pains and pain and abnormal sensations in areas served by swollen nerves that are nipped at exits from the spine.

That's only a theory at present - but studies are under way to try to confirm it. My own experience of people with the nerve disease peripheral neuropathy would tend to support it, so I look forward to hearing the results. Meanwhile, we may be at the birth of a new science - meteorological medicine. You read about it first here!

Converted for the new archive on 30 June 2000. Some images and formatting may have been lost in the conversion.