Scientists and health experts are working to gather more data and evidence on how climate change affects diseases and other aspects of health. Statistics are hard to come by.

Between 1997 and 2006, weather-related disasters killed an average of 71,000 people a year, according to the Brussels-based Centre for Research on the Epidemiology of Disasters. But it's impossible to say to what extent those disasters were caused by climate change.

People often cite a World Health Organisation estimate that the effects of climate change since the mid-1970s may have caused over 150,000 deaths in 2000. WHO says the impacts are likely to increase in the future.

On the positive side, global warming could bring limited local benefits, such as fewer winter deaths in temperate climates and increased food production in high-latitude regions. But the consequences are likely to be overwhelmingly negative, particularly for the world's poorest communities.

Who's most at risk? Besides those without access to health services, WHO singles out people living in small island developing countries, mountainous regions, areas where water resources are already stretched, megacities and coastal zones in developing countries - like India and Bangladesh.

One major problem is that the health consequences of predicted declines in freshwater supplies and crop yields, sea-level rises and displacement caused by dwindling natural resources are hard to measure - and we may not know these for several decades. They include malnutrition and illnesses caused by lack access to clean drinking water and sanitation.

The first detectable signs of the impact of climate change on human health are likely to be alterations in the geographic areas affected by vector-borne diseases like malaria and dengue fever - and their seasonality. Here are some of the potential medical effects:

• Wetter, warmer weather could take malaria-carrying mosquitoes into previously cool zones like the highlands of Tanzania and Rwanda, although they could disappear from other areas. Scientific modelling cited by WHO shows that global temperature risk of 2 to 3 degrees centigrade would increase the number of people at risk of malaria by around several hundred million, and lengthen the transmission season.

• Increased rainfall in hot areas is likely to boost dengue fever and Rift Valley Fever - also carried by mosquitoes - and leishmaniasis, a parasitic disease that causes blindness.

• Hot, fetid conditions encourage cholera, a waterborne disease which often spreads when floods spread drinking supplies with sewage, as well as diarrhoea and other stomach infections.

• Meningitis, passed through the air by coughing, sneezing and breathing, could increase in hot dusty regions such as the African Sahel if climate change brings more arid conditions.

• More frequent and intense heatwaves could increase deaths and cases of heat stress and heatstroke, particularly among young, elderly and frail people in urban centres. WHO says evidence - including 70,000 excess deaths from the 2003 heatwaves in Europe - shows this is already happening.

• Smog, air pollution and higher pollen levels could lead to more asthma and respiratory infections.

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