This book by notable popular science author Carl Zimmer details the primarily unseen world of parasites. Beginning with a discussion of why parasitism inspires such universal fascination and revulsion, and concluding with a slightly reaching analogy describing human beings as pests leeching off the Earth, Zimmer covers a wide range of topics within parasitology, biology, and evolution.

Zimmer has obviously performed extensive research on his subject, which the reader can duplicate with the aid of his helpful selected bibliography. He relates case studies of many specific parasite species, from the well known Plasmodium which causes malaria by infecting human red blood cells, to the parasitic barnacle of genus Sacculina, which invades the body of a crab, sterilizes it, and alters the host's behavior such that the infected crab actually nurtures the barnacle's eggs instead of its own -- even if the crab is male. These tales are often gruesome, and always fascinating.

In addition to gleefully expounding upon the gross-out factor inherent within parasitology, Zimmer also explains important scientific concepts introduced to the fields of biology and ecology by researchers studying parasites. He describes the scientists who struggled to understand parasite behavior -- how does a parasite who enters the host through its digestive tract find its way to other organs or systems where it will take up permanent residence? The original idea was that they follow concentration gradients, moving toward higher concentrations of some key chemical that is secreted by their target organ. However, the environment within a host differs from that of, say, an open ocean -- the host organism's job is to maintain uniformity, homeostasis, within its organ sysems, and to make sure that various chemical secretions stay where they belong -- not to spread them throughout the body leaving faint scent trails for parasitic worms to follow. Instead, it was found that internal parasites perform specific behaviors when exposed to location-specific chemical cues -- perhaps reacting to a liver secretion, even if they do not ultimately infect the liver. As they drift through the body, they await exposure to these cues, at which point they begin sudden swimming, thrashing, or burrowing behaviors. Performed in the right stepwise sequence, these behaviors ultimately lead the parasite to its final destination, through a process that had been unknown in conventional behavioral zoology.

Zimmer's personal fascination with evolution shines through in the book, as well. He mentions theories that attempt to explain sexual reproduction -- that much-beloved source of variability that drives evolutionary processes in you, me, the birds, and the bees -- as a result of organisms attempting to boost their resistance to parasite infection. This "arms race" between host and parasite has been described as the Red Queen effect, in which both parties evolve as fast as they can just to stay in the same place. Parasites evolve toward greater infectious capacity just as quickly as hosts evolve stronger resistance. (For more on this, try reading The Red Queen by Matt Ridley.) The author also details some of the more creative mechanisms that parasites have evolved in their pursuit of infectiousness. Some of these include such profound alterations of behavior as those seen by crabs infected with Sacculina, taken to an even more chilling level: there are seemingly parasites which infect an animal, then subsequently alter its behavior so that it is more likely to be eaten by the parasite's final host. The parasite Toxoplasma, for example, can infect mice, but its primary host is a cat. (It can also infect the human brain, producing toxoplasmosis -- a condition not without its own behavioral effects on the host.) Scientific studies have revealed that, among other things, mice infected with the parasite exhibit less aversion to the smell of cat urine -- typically a powerful deterrent to all rodents. It follows that these mice would be slower to flee from a cat, and thus more likely to fall prey to one, which is just what Toxoplasma "wants." This particular type of behavioral alteration has come to be known as the theory of Parasite Increased Trophic Transmission (PITT), and is believed to have strongly influenced the evolution of parasites whose life cycle depends upon more than one host species.

Overall, I think this book is a great read. I found Zimmer's writing to be entertaining and approachable. As a student of biology, I came in with a bit more background knowledge than the typical layperson might have, but I still learned a lot of new things. The book isn't all hard science -- Zimmer references pop culture phenomena like the series of Alien movies and The X-Files to provide evidence of widespread morbid fascination with parasitism, for example. However, most of the text involves descriptions of various scientists and their research in parasitology, which opened a window into a world I'd never seen before. If you've any interest in reading a work of popular science, I'd heartily recommend this book. Thanks to Carl Zimmer's fascinating portrayal of parasitology, I may just apply to a few graduate schools in the field.