Parasites, the stuff of many people’s nightmares, are a biologist’s dream—superbly adapted creatures that have evolved sophisticated strategies for living off their hosts.
Discover editor Zimmer (At the Water’s Edge, 1998) describes the parasites’ lifestyles in vivid detail. His subjects range in size from the protozoan Plasmodium (which can fit inside a human red blood cell) to tapeworms, which can grow 60 feet long. Living inside another creature’s body requires developing elaborate ways to dodge the immune system, from hiding in cysts to releasing tame viruses that decoy defenses from the actual threat. Some parasites can modify the behavior of their intermediate hosts, making them more vulnerable to the predators that are their final hosts: Toxoplasma, which passes from rats to cats, turns off a panic mechanism triggered by the smell of cat urine, so the rats no longer instinctively avoid their feline hunters. Many parasites sterilize their prey, diverting energy from reproductive activity to the creation of food for the parasite. Parasitologists believe that this sort of behavior, making some infected animals 30 times more likely to be eaten, has a profound effect on the balance of predator and prey species in the wild. But to most readers, the real meat of the book will be its description of the ways in which parasites affect the human race. The biggest surprise: rainforest Indians in Venezuela, commonly infested with intestinal parasites, are almost entirely free of asthma. Scientists speculate that, without parasites to repel, the immune system turns its attention to otherwise minor irritants such as dust mites and cat dander. As with so many other apparent advances, the cure for one disease may well be the cause of another.
An eye-opening perspective on biology, ecology, and medicine—well worth reading, even if the subject makes you squeamish.