The killer form of malaria is caused by a parasite, Plasmodium falciparum, which is transmitted by the bite of the Anopheles gambiae mosquito. Malaria parasites grow and develop inside the host mosquito. Before they can be transmitted to humans, they need warmth to complete their growth cycle. This explains why malaria mainly occurs in tropical climates. Besides temperature, malaria transmission is influenced by the abundance of mosquitoes and the propensity of the Anopheles gambiae species to bite humans rather than animals. Sub-Saharan Africa's ecology is ideal for transmission, which accounts for the intensity of transmission there.

Dichloro-diphenyl-trichloroethane (DDT) is a chemical that can be sprayed lightly inside human dwellings to combat mosquitoes. One treatment lasts for up to a year. Scientists who study malaria have demonstrated through research and field work that DDT is effective against as much as 97 percent of a population of mosquitoes that would otherwise transmit disease inside a home.

Despite its proven value in the fight against malaria in the 1940s and 1950s, DDT ultimately was banned from use in the United States and many other countries. The arguments against using DDT were not without merit. The same properties that make this chemical an effective tool against mosquitoes make it potentially toxic to beneficial insects, animals, and perhaps humans. Farmers had begun using DDT to protect their crops, which meant they were spraying it over entire landscapes—not in a safer, more targeted fashion.

Like many pesticides commonly in use, DDT is neurotoxic; that is, it can damage or destroy nerves or nerve tissue. It has also been linked to cancer and other illnesses. Another property of the DDT molecule is that it persists for decades without breaking down. Thus, when an animal ingests it, the chemical accumulates in the animal's fatty tissue. This means that animals at the top of the food chain, which prey on those below it, will ultimately carry the highest dose. This can endanger an entire population of animals.

Over the years, numerous studies have investigated the effects of DDT on human health. Yet these studies have failed to produce conclusive evidence that DDT is harmful to human health. Still, based on the effects it has shown in animal studies, the U.S. Environmental Protection Agency has classed DDT as a probable human cancer-causing agent.

Public health officials concerned with fighting malaria where it is still an active problem, such as in Africa, have a difficult choice to make. No other option to combat mosquitoes and the spread of malaria has been as effective, long-acting, and inexpensive as DDT. The use of bed nets and insect repellants simply do not control the source of the problem: the mosquito population. In places where DDT was widely used, some mosquito species have developed a resistance to the DDT, so it is no longer as effective. However, because DDT has not been as widely used in Africa, there is reason to believe it could make a difference in at least reducing the incidence of—if not eradicating—this killer disease.

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