Weather: El Nino is being blamed for a wide range of climatic catastrophes in the southern hemisphere - fires, floods, drought, hurricanes, landslides and crop failure among them. As if that were not enough, we are now being warned of another danger: the threat of disease.

With all the problems that El Nino has been causing, it is easy to forget that the worst is probably still to come. It was, after all, the Peruvians who gave this build-up of hot water in the Pacific its name, calling it "El Nino" after the Christ Child, because it tends to peak around Christmas. But as the US prepares for possible hurricanes, or floods, or drought, or whatever else may turn up, the Pan American Health Agency issued a statement yesterday warning of hazards to health. "If we see a serious El Nino, which is predicted, we'll undoubtedly see health effects," said Dr David Brandling-Bennett.

The basic problems are twofold: floods will contaminate water supplies by mixing waste and sewage with traditional sources of drinking water, as well as leading to increased breeding among disease-carrying pests, while droughts will deplete freshwater reservoirs and diminish food supplies. Floods also tend to drive disease-ridden rodents out of their homes, and lead to conditions in which mosquitoes thrive.

The Paho advises vulnerable nations of both North and South America to invest in preventive measures now, which will result in improved health even if the effects of El Nino turn out to be milder than feared.

The general effects of climate on human health were well aired in a chapter by Anthony J McMichael in Climate Change 1995, the report of the Intergovernmental panel of Climate Change, and it makes chilling reading. Here's one of the high-confidence predictions: "If extreme weather events (droughts, floods, storms etc) were to occur more often, increases in rates of death, injury, infectious diseases, and psychological disorders would result." More specifically, mathematical models of climate change predict that, within the next 75 years, the percentage of the world population living within the potential malaria transmission zone will increase from 45 to 60. According to one WHO calculation, the number of cases of malaria each year is currently 300-500 million, but the number of people potentially at risk could rise to 2,400 million.

And that comes on top of problems that are already with us: "Many studies ... have observed a J-shaped relationship ... between daily outdoor temperature and daily death rate: mortality is lowest within an intermediate comfortable temperature range. The graph is not symmetrical; the death rate increases much more steeply with rising temperatures, above this comfort zone, than it does with falling temperatures." So although warmer climates may bring a reduction in winter deaths from cold, it is unlikely that this will offset the heat-related deaths in the summer. Then there is schistosomiasis, which is spread by the water snail, and onchoserciasis, spread by the black fly, and dengue and yellow fever - all of which are predicted to increase dramatically with climate change.

On the Today programme on Radio 4 on Tuesday morning, I heard (though, sadly, did not note who said it) a splendid line to the effect that there is no such thing as bad weather, only being dressed wrongly for it. In a more general sense, most potential health hazards of bad weather are similarly avoidable. We just have to make sure we're dressed properly for them.