Vaccines also run the risk of tearing down our hard-earned ability to immunize ourselves. Say that the navy vaccine against the liver stage of malaria works, and that it is decided to inject it into millions of children around the world. Now say that the vaccine works brilliantly for a few years. Now say that countries let the program lapse because of civil war or because speculators sell off the national currencies. Or, if you like, say that a mutant strain of malaria sweeps through, different enough to keep T cells trained on the vaccine from recognizing it. Now the people would have no protection in their livers, and wouldn’t have had the opportunity to build up their own resistance to the blood stage of the parasite. The vaccine could then conceivably cause more harm than good.
For some parasites, it may actually make more sense to find a better coexistence than to try for eradication. In schistosomiasis, for example, the adult blood flukes themselves don’t cause much harm. They’re so well cloaked from the immune system that they don’t trigger a damaging attack, and they don’t drink much blood. It’s their eggs that are trouble, as the immune system forms giant balls of scar tissue around them in the liver. Among the many signals that immune cells trade, one has the ability to stop them from making these granulomas. Scientists have found that if they give an extra dose of this signal to mice with schistosomiasis, they don’t destroy their own livers. Conceivably, this kind of medicine could save us—not from parasites but from ourselves. Another strategy could be to keep the blood flukes from mating. Scientists have discovered that males attract females with a chemical signal. If people were vaccinated so that their immune system could destroy that signal, blood fluke love would be foiled, and no eggs would be made.
Coexistence with parasites might also be possible if we could tame them. The severity of a disease caused by a parasite has a lot to do with its evolutionary options. If a virus’s best chance for survival requires it to kill its hosts quickly, it will probably evolve into a lethal strain. But the opposite is also true: if the virus has to pay a heavy price for being virulent, more benign strains will win out. For well over ten thousand years, we’ve actually been managing a lot of evolution as we’ve bred plants and animals for the qualities we desire—docile cows, for example, and sweet apples. One of the architects of the theory of virulence, Paul Ewald of Amherst College, has proposed doing the same thing with parasites in order to fight diseases. It’s actually not hard to domesticate a parasite. In many parts of the tropics, for example, public health campaigns are supplying people with screens and bed nets to keep malaria-carrying mosquitoes from biting them as they sleep. The campaigns will save lives not only by preventing mosquito bites, Ewald suspects, but by forcing the Plasmodium inside the mosquitoes to evolve into a gentler form. As it becomes less likely that a parasite can get from one host to the next, it becomes unwise, evolutionarily speaking, to kill a host.
Eradicating parasites may even create new diseases. Colitis and Crohn’s disease affect 1 million Americans today. In both cases, a person’s own immune system violently attacks the lining of the intestines. The inflammation it triggers ruins a person’s digestion, and sometimes a surgeon may have to cut out a length of the damaged bowels. Both diseases can torment a person for a lifetime, and so far there’s no cure for either. Yet, as common as they are today, you can’t find any record of colitis or Crohn’s disease before the 1930s. The first cases in the United States turned up in well-to-do Jewish families in New York City, which made doctors think they were hereditary diseases. But then whites who weren’t Jewish started getting them. Still doctors thought the diseases were hereditary because hardly any blacks fell ill. But in the 1970s, blacks started getting the diseases as well. Looking outside the United States, you can see another peculiar pattern. In the poorer countries in the world, the diseases are practically unheard of. Yet, in Japan and Korea, two countries that have quickly gone from poverty to wealth, there are now epidemics of colitis and Crohn’s disease.
Some scientists think that the spread of these diseases was caused by the eradication of intestinal worms. The idea certainly fits their history. In the United States, they appeared first in affluent people in cities—the people, in other words, who would have been the first to be cleared of tapeworms, and other worms living in their bowels. Later, when blacks began to emerge from poverty and moved to cities as well, they also fell ill. Intestinal parasites are still common in most of the world, but in countries where they’ve been recently eradicated, colitis and Crohn’s disease have followed fast. Even farm animals are starting to get bowel diseases as they’ve been getting treated with antiworm medicines like Ivermectin.
Humans may have been protected from diseases like these by the interplay between their immune systems and intestinal parasites. Parasitologists have found that intestinal worms can nudge the immune system from a poison-spouting, cell-engulfing frenzy to a gentler sort of attack. In this mellower mood, the immune system can still keep bacteria and viruses in check, but the parasitic worms can live unmolested. This arrangement benefits the host as well. When parasitic worms are abundant, it would be dangerous to attack them over and over again. But then, in an evolutionary blink, a few hundred million people lost their parasites. Without their soothing influence, some people now swing too far the other way, their immune systems unable to stop attacking their own bodies.
In 1997, scientists at the University of Iowa put this idea into startling practice. They picked out seven people with ulcerative colitis and Crohn’s disease, who had gotten no relief from any conventional treatment. They fed them eggs from an intestinal worm that normally lives in an animal, one that wouldn’t cause any disease of its own in a human gut (the scientists are still keeping the species a secret until they’ve finished their research). Within a couple of weeks the eggs had hatched, the larvae had grown, and six out of the seven people went into complete remission.
Parasite-free living may also be responsible for the rise of other immune disorders, such as allergies. Twenty percent of the population of the industrialized world suffer from allergies, but elsewhere they’re hard to find. Since it’s dangerous to generalize from country to country, an immunologist named Neil Lynch has done fine-grained studies of this pattern in Venezuela. He looked at people in upper-class homes with running water and toilets, and compared them with poor Venezuelans in slums. While 43 percent of the upper-class people had allergies, only 10 percent had light infections from intestinal worms. Among the poor, there were half the allergies as in the upper classes but twice the worms. And when Lynch studied Venezuelan Indians who live in the rain forests, the pattern was even starker: 88 percent were infected with parasites, and they had no allergies at all. Without parasitic worms exerting their influence, our immune systems may be prone to overreacting to harmless bits of cat dander and mold.
To fight these diseases, we may need to acknowledge our long marriage to parasites. That’s not to say that people with colitis should be eating Trichinella eggs unless they’d enjoy a long, agonizing death as the parasite worked its way into their muscles. But the chemicals that the parasites use to manipulate our immune systems may offer protection from modern life. Perhaps some day, along with polio vaccines children will get parasite proteins, so that their immune systems will be trained not to fly out of control. It would be a supreme final twist to the story of parasites in humans. They may not always be the disease. In some cases they may be the cure.