Malaria is one of the most debilitating diseases. Its sweep is awe-inspiring, affecting one person in 20 worldwide. Up to 500 million a year become infected and two million die from it annually, mostly in sub-Saharan Africa. The overwhelming proportion of deaths are young children and pregnant women. Among communicable diseases, it is the second most deadly after tuberculosis.
While the vast majority of cases have been in tropical and sub-tropical countries, there is a dramatic increase in the disease in Europe and north America.
What enables a disease that is at home in countries near the equator to thrive in more temperate regions? The carrier of the malaria parasite is a highly adaptable insect, the Anopheles mosquito, which lays its eggs in clean water and can infect many people in its short life. Until relatively recently, malaria was endemic in large parts of North America and Europe.
Oliver Cromwell died of it in the marshlands of East Anglia in 1658. There were epidemics in the Russian Arctic town of Archangel in the 1920s and in the Netherlands in 1946. Thanks to concerted campaigns in postwar Europe and elsewhere which were based mainly on the now prohibited pesticide DDT and land drainage of stagnant water, malaria was eradicated for nearly 30 years from those regions. In 1975, the World Health Organisation (WHO) declared Europe malaria-free.
But the world was a different place in the mid-1970s. The collapse of the Soviet empire since then has had a profound impact on health care for eastern Europe and Asia. In the past decade there have been outbreaks of malaria in the Russian federation, Afghanistan, Tajikistan and Azerbaijan.
Turkey, Greece and Italy have also been affected.
There are a number of factors for this. In the former Soviet regions, there is only rudimentary health care. Public health programmes such as malaria control have ended and the understanding of the disease that existed during the anti-malaria campaigns of the postwar years has disappeared.
But in western Europe and America, the main factor contributing to the re-emergence of malaria is importation. People migrating from one region to another bring diseases with them. Most of the cases of malaria in western Europe contracted the disease in regions where it is endemic. Over the past 30 years, the number of imported cases across Europe has increased eight-fold, to more than 16,000 a year - and that figure takes into account the fact that only between a quarter and a half of cases are notified to health authorities. The UK alone has 2,000 imported cases every year.
Another way the disease is imported is by what is called airport or luggage malaria: just as people bring diseases with them, so aeroplanes transport insects thousands of miles. About 75 cases of airport malaria have been reported in Europe since the phenomenon was first identified in 1977. Those affected range from cabin crew and passengers to airport staff and people living nearby. A 13-year study of insect stowaways on American aircraft conducted by the US Public Health Service found 92 species of mosquito on board, 51 of which were not native to the US. Changing planes or stopping over in a malarious country can also expose passengers to infection.
Public works can also play a part in the transmission of the disease. In sourthern Turkey, for example, in the past decade, irrigation networks, more than a dozen dams and 19 hydroelectric power plants have furthered agricultural and economic development, but they have also created vast areas of standing water which are mosquito magnets. More than 15 million people now live in parts of the country where malaria is endemic and a further 28 million live in areas at high risk.
Some scientists believe global warming is contributing to a rise in the mosquito population. The British chief medical officer published a report two years ago saying that climate change could re-establish malaria-carrying mosquitoes here by 2050. Another report from the London School of Hygiene and Tropical Medicine says climate change is resulting in an increase in child deaths from malaria in developing nations.
But others reject these assertions, and this is where malaria has become a political argument, dividing scientists and health policy makers. There is substantial evidence showing that malaria is increasing as a result of banning DDT, which is toxic to animals, birds and insects. But it is also the most effective known weapon against mosquitoes, reducing malaria and other mosquito-borne diseases by up to 90 per cent.
The huge anti-malaria programme that was conducted throughout Europe and the southern US after the Second World War involved the use of DDT, in conjunction with land drainage. This chemical was also used in several developing countries and is estimated to have saved 500 million lives over 20 years. But the 1962 publication of Silent Spring, Rachel Carson's expose of the effects of insecticides on human health and the environment, shifted public health policy and led to a banning of DDT by western countries in the 1970s.
This, says the pro-DDT lobby, has been the major factor in the re-emergence of malaria in the past 30 years. Richard Tren, head of the independent organisation Africa Fighting Malaria, calls it "eco-imperialism". Backed by health policy heavyweights such as the American Council on Science and Health, there has been a call for the reintroduction of DDT in the absence of clear evidence to show that it harms humans. These scientists say that controlled use of DDT is no more detrimental than approved insecticides.
Nowhere else has the DDT ban had such an impact as in Africa. Less than a quarter of the continent's 100 malarial countries still use it. American and European-based international aid donors, on which many African countries depend, insist that the war on malaria must be DDT-less for environmental reasons. South Africa stopped spraying DDT in 1996 and saw malaria increase by 150 per cent. Since it resumed spraying in 2000, the number of cases has reduced considerably. When Sri Lanka banned the chemical in the mid-1960s, malaria increased from 17 cases to 2.5 million in five years. Mozambique, where there is a ban on DDT, has a malaria infection rate between 20 and 40 times higher than its southern DDT-spraying neighbour Swaziland.
Environmentalist objections to DDT have softened in the past few years. In 2000, a group of 120 countries that had come together to draft a UN treaty banning "persistent organic pollutants" agreed to exempt DDT as long as it was employed only to control malaria. Although it will still require countries to record and report on their use of the chemical, the treaty is seen as a step in the right direction.
But prevention is undoubtedly the best cure in the fight against malaria.
And the controlled use of DDT must be the best answer now.