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The toll taken by these parasites is overlooked for several reasons. One is that it happens mostly to the poorest people in the poorest countries. Another is that many of these parasites aren’t outright fatal. Although 1.3 billion people carry hookworm, only 65,000 people actually die of it each year. But the effects of chronic infections with parasites are still devastating, leaving people listless and undernourished. Parasites like hookworm and whipworm make it hard for children to learn in school; all it takes is a dose of antiwhipworm medicine to make some slow children bright again.

Epidemiologists have tried to quantify this sort of loss with something they call the disability-adjusted life year. Simply put, this unit measures the estimated value of the years of healthy life lost to a disease. It is a grim exercise in statistics, replete with the cold-hearted calculations of labor—getting blood flukes at age twenty-five counts for much more than at age fifty-five. Depending on how bad a disease is, a year still living counted as only a fraction of a life lived parasite-free. Roundworm may slow down a child’s growth, but if it’s caught in time, the condition is reversed and the child begins to grow again. Left too long, though, roundworm can leave the child stunted into adulthood. When considered in this way, parasites are a staggering drain on life. Malaria robs the world’s population of 35.7 million life-years every year. Parasitic worms of the gut—hookworms, roundworms, whipworms most importantly—are far less fatal than malaria but actually rob more life: 39 million life-years. Taken together, the leading parasites destroy almost 80 million life-years a year, almost twice as many as those claimed by tuberculosis.

In the United States, most people aren’t aware of the damage that parasites wreak (or even know what these parasites are) because they’re such a small threat to their own health today. It wasn’t always the case. Most Americans don’t know that in the 1800s, malaria’s range swept all the way up the Great Plains into North Dakota, or that in 1901, a fifth of the population of Staten Island carried the parasite. Most don’t know that people in the southern United States once had a reputation for being lazy and stupid because so many of them were being drained by hookworm. Most don’t know that in the 1930s, 25 percent of the pork sold in the United States carried Trichinella.

The United States no longer has to worry about these parasites, but not because anyone invented a magic bullet. They’ve been overwhelmed by the slow, dogged work of public health, of building outhouses, of inspecting food, of treating infections to break the cycles that parasites had taken for thousands of generations before. There’s still plenty of life in this simple approach. Consider the hideous case of guinea worms. Even at the middle of the twentieth century, guinea worms were fantastically successful parasites. One estimate in the 1940s had them crawling out of the legs of 48 million people every year. Today there is still no vaccine for guinea worm disease, nor is there even a medicine known to work against it. But in the early 1980s, public health workers began a campaign that may eradicate it from the face of the Earth.

Their strategy was simple. They made people in the guinea worm zone aware of the parasite’s ways. They helped set up wells in some places and issued cheesecloth in other places to filter out parasite-carrying copepods from pond water. They stopped people from helping the guinea worm complete its life cycle by putting bandages on the abscesses the parasites formed. As the guinea worms were spooled out of their hosts, their hosts were kept away from water. In a matter of years the guinea worm population started to crash. In 1989, there were 892,000 reported cases (the actual cases were probably far more); in 1998, the number had dropped to 80,000. Guinea worms disappeared from Pakistan altogether in 1993. It’s conceivable that within a few years, guinea worms will be completely wiped out. After smallpox, guinea worms would then become only the second disease to have been eradicated in the history of medicine.

Two other pernicious parasites also have life cycles that make them good candidates for eradication. One is Onchocerca volvulus, the worm that travels in black flies and causes river blindness. Seventeen million people carry the parasite, mostly in Africa. Short of wiping out all the flies or issuing insect spray to all Africans at risk, there would be no way to keep people from getting infected. Like guinea worms, O. volvulus has no vaccine, but it does have a partial cure. Sheep ranchers give their animals a drug called ivermectin to cure them of intestinal worms. Ivermectin seems to paralyze the worms so that they can’t feed or swim, and they get flushed out of the body. Parasitologists have discovered that ivermectin actually works effectively against many other parasites, including O. volvulus. If a person with river blindness takes the drug, the baby worms that wander through the skin die. It’s not a complete cure, since the adult worms are left snuggled happily in their nodule, where they can give birth to thousands more baby worms. But it’s the babies that cause the worst symptoms of the disease—the agonizing itchiness and the scarring of the eye that leads to blindness. Researchers found that if an infected person took one pill once a year, he would be free of the babies. Since an adult worm lives ten years, he would have to take it ten times to be completely cured. The pharmaceutical colossus Merck has donated as much ivermectin as will be necessary to cure the world of river blindness, and 100 million doses have been handed out so far.

More recently, parasitologists have found that ivermectin can work as efficiently against the filiarial worms that cause elephantiasis. The filarial worms have essentially the same life cycle as O. volvulus, and the same susceptibility to ivermectin. The project is far more ambitious—120 million people throughout much of the tropical world are infected. If these researchers should be successful and if these three parasites are destroyed, the world should honor them for waging these campaigns. We can look forward to a time when people will have a hard time believing that there was anything on Earth that could have caused human agony in such elaborate ways. They will be the dragons and the basilisks of the twenty-second century.

Yet, in their vulnerability these three parasites are exceptions rather than the rule. Many others thrive on the poverty that most of the world lives in, and it takes more than some good intentions to stop them. Schistosomiasis is easily curable if you’ve got the twenty dollars to buy the drug praziquantel. If you’re too poor to afford it on your own but someone gives it to you free, the chances are you’ll just get sick again because you have to get your water from a pond instead of a clean well. And often the supposed cures for poverty make the lives of parasites easier. When giant dams are built and submerge vast regions of dry land, they create new homes for the snails that carry blood flukes, and new epidemics of schistosomiasis reliably follow.

The most important reason that parasites do so well today is that they evolve. Parasites are not life’s dead ends, as was once thought; they are continually adapting to their circumstances. Not only has malaria been forcing us to evolve; it has been evolving to adapt to us. And after adapting to natural human defenses for many thousands of years, Plasmodium now simply has to go up against drugs rather than some new T cell receptor.

Before the 1950s, the malaria a person contracted anywhere in the world could be treated with a few doses of the benign drug chloroquine. Chloroquine cures malaria by turning Plasmodium’s food into poison. As Plasmodium feeds on the hemoglobin in red blood cells, the parasite chops off the arms of the molecule, leaving behind the iron-rich core. This core is dangerous to the parasite, because it can lodge in Plasmodium’s membrane and disrupt the flow of molecules in and out. The parasite neutralizes the poison in two ways. It strings some of the molecules into harmless hemozoin; the rest it processes with enzymes until it can no longer react with the membrane.