Because it is so deadly, the malaria parasite has wielded surprising influence over the way human beings have evolved. Sickle-cell anemia, an excruciating and usually fatal genetically transmitted blood disorder, offers the most dramatic example. Historically, victims of sickle-cell disease, who have inherited the sickle-cell gene from both parents, were generally sick much of their lives and tended to die young, often before they had the opportunity to bear children. This should doom sickle-cell in terms of natural selection, which by definition selects for characteristics that ensure that an organism will live long enough to reproduce itself. But the sickle-cell trait, which occurs in people who get the gene from just one parent, is not lethal, and in fact confers partial protection against malaria. People who have the sickle-cell trait may get sick with malaria, but they are unlikely to die of it. In some parts of Africa a quarter of the population has the sickle-cell trait -- an almost incredibly high proportion considering that a double dose of the gene is fatal. The evolutionary fact that human beings have taken on a lethal blood disorder in exchange for partial protection against malaria shows the unprecedented power of the disease.
Malaria inspires such fear that forty years ago the World Health Organization targeted it for global eradication. The United States spent heavily on the campaign, in 1958 committing $23 million a year for five years to the WHO program. Worldwide investment in the project ran into the hundreds of millions of dollars, with some Third World countries kicking in 30 to 50 percent of their health-care budgets. The main thrust of the effort was the spraying of powerful, long-lived insecticides into the interior walls of homes. Workers tramped through villages in North Africa and Asia with spray guns loaded with DDT. The target was the Anopheles gambiae mosquito, which carried P. falciparum.
Hut after hut was sprayed, and untold legions of A. gambiae were ambushed. Malaria rates went down, and hopes for public health soared. But the optimism was short-lived. It soon became clear that spraying was most effective in areas that were only marginally malarious -- areas such as Egypt and southern Europe, where the parasite had only a slippery hold. Meanwhile, for complex reasons, mosquitoes where malaria was solidly endemic started showing resistance to the insecticides.
In 1969 the Global Eradication of Malaria Program was replaced with a global malaria-control strategy. But in 1975 reported malaria incidence was far higher than what it had been fifteen years earlier. Harold Varmus, of the NIH, recalls the impact in India as particularly dramatic. "When I was working there in 1966, I saw almost no malaria," he says. "But when I returned in 1988, there was a raging epidemic. This made a real impression on me. We thought we'd licked malaria in India, but of course we were terribly wrong."
The WHO now estimates that malaria kills as many as 2.7 million people worldwide each year (roughly twice as many as AIDS), and that it sickens as many as half a billion. Children infected repeatedly with malaria over a period of about five years generally develop a partial resistance that allows them to carry the parasite in their blood without getting deathly or even overtly ill. Adults develop this partial immunity too, in somewhat less time. But this does not mean that they can no longer be infected; it means only that their bodies have learned to keep the symptoms in check. Malaria parasites have a voracious appetite and in just a few hours can suck as much as a quarter pound of hemoglobin out of the red blood cells of an infected human being. Hundreds of millions of African children and adults are chronically infected with malaria, and are anemic much of the time. Bernard Nahlen, a physician who is studying malaria in Kenya, told me that one in twenty children in the villages surrounding his clinic are so anemic that were their blood tested in the United States, they would be rushed to a hospital for emergency transfusions. Another American physician in Kenya, Patrick Duffy, says that when he has treated Africans with drugs to clear their blood temporarily of malaria parasites, the patients report feeling as they have never felt before -- that is to say, well.
ON the main road leaving Dakar through the savanna of western Senegal traffic thins and the landscape thickens with sorghum, melon, and peanut farms. Manioc sprouts in bushy clumps, and nomadic herders thread flocks of bony goats and long-horned cattle through clumps of mango and baobab trees. Lamine Diawara, a physician, an entomologist, and a commander in the Senegalese army, pulls our pickup truck over to ask directions from a group of people selling oranges by the side of the road. One of them, a man of about twenty in a tattered knee-length tunic and a headdress, swings first a canvas sack of peanuts and then himself into the bed of the pickup. Diawara shrugs and drives on. The truck he has borrowed to make the journey is finicky and slow; we pass horse-drawn carts but no cars. Dignified in his sharply pressed military uniform, Diawara is uncomfortable with silence, and his voice is soft but urgent. Since morning he has spoken of almost nothing but malaria. "Westerners think only of HIV," he tells me. "HIV is horrible, of course. But it is malaria that keeps Africa down."
Diawara is taking me to a village where he often comes to observe and treat patients. The village is a cluster of thatched huts some distance off the main road, on a dirt track that dissolves into a grassy path. Here most of the women have infants slung on their backs, as do some girls who appear to be as young as five. Young men relax in the shade while their wives pound sorghum and sort seeds. The children crowd Diawara, who strides off to seek the elders, five of whom emerge beaming from one of the huts, hands outstretched in greeting. Speaking in Woloff, Diawara asks permission to show me around, and they volunteer to serve as guides. I ask if they have been ill lately. The head elder nods, and says he has suffered from malaria four times this season. The others indicate that they, too, have been afflicted. We visit the village clinic, a cement-block hut hung with posters promoting "safe love" in English and Arabic. Inside there are no medical supplies, no equipment, and no lights. There are no medical personnel; a teenager trained in recognizing malaria symptoms comes in from time to time to hand out anti-malaria medications. This is the best that can be hoped for in rural areas, because anti-malaria drugs are costly; often there is no money for them. Another problem is that malaria is diagnosed not by its symptoms, which are ambiguous, but by checking for parasites in the blood, which requires a microscope. There probably isn't a microscope within miles of this place. When fever strikes in Africa, it is often assumed to be malaria, and if drugs are available, they are simply administered. A child may die of measles or meningitis while being treated for malaria.
I ask for a look inside a home and the head elder takes me to his. The single room is clean and cool, and a tightly made double bed fills most of the floor space. The only other object is a billowing mosquito net, whose corners are tucked neatly around the bedposts. Bed nets impregnated with insecticide are widely touted as a cheap and easy way to prevent malaria in the tropics. Many articles and even a book have been written advocating the use of bed nets, and the WHO devotes considerable space on its Web site to the health benefits of nets. I ask the elder if he uses his. He untucks the net, poses beneath it, and asks that I take his photo. Yes, he says, he uses the net. But still he gets malaria. He loses about two or three weeks a year to the disease. He says he is tired much of the time. The other elders nod, smiling. They, too, they say, are tired.
"Nets might help, but they will not cure the problem," Diawara tells me later. We are back in the truck en route to the nearby city of Thiès. "Bed nets reduce exposure, but they cannot stop it. Not all mosquitoes bite people in bed." I ask Diawara if he uses a net, and he says he doesn't. Nor, he presumes, does the elder who posed, or any of the people we just met in the village. "The elder wanted to show respect to a Westerner," he says. "The West brought the nets. But people here do not believe that they work, and most of us do not use them. Nets keep out the breeze, and this is a very hot country."