El Niño, the weather cycle that can cause freak weather from California to Cape Town every 2 to 7 years, is also thought to influence the incidence of several infectious diseases including malaria, dengue and cholera.

Now a report in The Lancet (3 February 2000) suggests that during the most recent El Niño event in 1997 and 1998, 200 per cent more children than usual were admitted to one hospital alone for non-cholera-related diarrhoea in Lima, Peru. This is estimated to have cost the hospital $277,000.

The effect seems to be caused mainly by the temperature increases that El Niño brings to Lima. Far more children were treated for diarrhoea in winter, for instance when the mean difference in daily ambient temperature averaged just over 4 degrees centigrade.

Robert Gilman of the Johns Hopkins School of Public Health, and his colleagues also found that in the months running up to the El Niño event, diarrhoeal hospitalisations increased linearly, by eight per cent for every one per cent rise in temperature. This does not happen during normal years.

"If our findings are reproducible," the team warns, "then diarrhoeal diseases may increase by millions of cases world-wide with each degree of increase in ambient temperature above normal."

Diarrhoea kills three million children under five every year. Hot weather favours bacteria such as E.coli and mosquitoes thrive too. Cool weather enhances the transmission of viral diarrhoea. So at intermediate temperatures, between 18 and 23 degrees centigrade (the kind of thing seen in a Peruvian 'El Niño winter') children may be exposed to everything: bacteria, viruses and parasites.

It is also possible that the raised temperatures associated with El Niño cause people to behave differently. They may wash and drink more -- both of which increase the transmission of diarrhoea, or they may even eat different foods.

El Niño is the warm phase of the 'El Nino/Southern Oscillation', a vast movement of warm water in the Pacific Ocean, brought about by a change in winds. As well as raising the temperature in Peru, it can bring storms to the California coast, droughts to Australia and both heavy rainfall and drought to southern Africa.

Given that El Niño's influence on the weather varies from country to country and even from region to region, its effects on health and disease will almost certainly differ accordingly. Even so, understanding weather effects such as this one is important for health service planning, and it could be even more important still, if global climate change occurs as projected.

For more information on El Niño, see NOAA's El Niño theme page.