If you have allergies or immune problems, one reason may be that as a child you failed to get a nematode. Nearly all of the several billion people living in developing countries house nematodes, but in developed countries these pale, parasitic worms are more scarce, rare even. The idea that the loss of parasites has had negative effects on our health is old. What is new is the growing understanding of the mechanisms underlying these effects. Nematodes could save your life—and now we are beginning to know how.
Several years ago while I was living in the Bolivian Amazon, I developed aching joints. I was in a jungle populated by people who had malaria, dengue, and the chronic problems of a hard life in a poor country: my complaints seemed silly. Still, my joints bothered me so I mentioned them one day to my neighbor over a dinner of jungle rat and rice. The neighbor, a local woman who, like most people in the world, extracted most of her medicine from the land around her, said she could help me. She took me to a tree surging with long, thin, reddish ants and told me to put my leg against the tree. The ants lived in the hollow center of the Palo Diablo (devil tree), which they were, by all appearances, prepared to defend. Reluctantly, I applied my leg. A dozen ants immediately swarmed up to my thigh and then up and, how do I put this politely, beyond. As they began stinging me, Maria said I could move away from the tree now. When I stopped screaming, I looked over at her. She was on her knees laughing—and my knee had stopped hurting.