Much has already been written about Carl Zimmer’s Parasite Rex, and I have to concur with the early reviewers: it’s a fascinating work. Not a novel, but it occasionally reads like one, especially if you’ve never contemplated the mysteries of parasites in all their various forms.

Probably you have not. Even the word “parasite” has a bad connotation in our society. Parasites are the spongers, the moochers, the lowlifes. Call someone a parasite and it’s clear you have nothing but contempt for him.

But that’s human society, says Zimmer. In nature, the role of the parasite may be considerably different. Modern thinking in the field of parasitology suggests that parasites might not be mere afterthoughts in natural ecosystems, but absolutely essential to them.

If that about-face isn’t wild enough, Zimmer has plenty more stunners to lay on you. The most mythologized aspect of modern parasitology is the idea that parasites can control the behavior of their hosts. Toxoplasma gondii, for example, can make mice hang around in areas that smell of cat urine — the mouse equivalent of spending a night alone in a haunted house. The eerie implication is that Toxoplasma can make mice want to get eaten, so that the parasite can be passed on to its preferred host: a cat.

Studies suggest that Toxoplasma can alter the behavior of human hosts, too. What’s more, Zimmer tells us, in some areas of Europe virtually 90 percent of people are infected with Toxoplasma gondii. Could this one microscopic parasite be influencing the behavior of entire human populations?

It’s an intriguing notion. To his credit, however, Zimmer cautions against anthropomorphizing these tiny invaders. A parasite like Toxoplasma gondii isn’t really an evil puppet-master. Its powers, though poorly understood, are probably attributable to nothing more than some simple magic on the molecular level — nature’s version of Prozac, say.

The thing to think about, Zimmer emphasizes, is the bigger picture. Parasites are virtually everywhere. Believe it or not, the majority of living species are parasitic. Each has some kind of impact on the other creatures in its ecosystem, even if it’s not explicitly tinkering with their brains. Viewed cumulatively, the impact of parasites on nature must be profound — so much so that eliminating parasitic organisms could put the natural order itself at risk.

The main weakness of Parasite Rex is that, as slim as it may be, Zimmer unfortunately seems to spend a lot of space reiterating the same ideas. To be fair, he points out that modern parasitology is very young, and part of his reason for writing the book is to popularize the field. If he can’t elaborate a certain point in more detail it’s often because we simply don’t know the answers yet. Still, as his chapters wore on, they sometimes seemed thin on real content — lacking names, dates, or hard science — and maybe better suited to shorter magazine articles until the facts get fleshed out more.

Zimmer returns to a favorite group of parasitic organisms again and again: blood flukes, Guinea worms, the microbes that cause malaria and African sleeping sickness. Obviously these are the “glamor parasites,” conjuring images of heroic doctors laboring in feverish jungle villages. But other, more commonplace parasites are completely ignored — head lice, for example, are never mentioned.

The book suffers somewhat from a semantic issue, also. Viruses and bacteria also live parasitically inside other living things, but they aren’t normally lumped into the grouping that scientists call “parasites.” Zimmer does point this out, but he doesn’t spend much time worrying about it. Where it becomes a problem, though, is in the later chapters, where he speculates about the big-picture effects of parasites — how they might keep animal populations in check or even spur evolution. He seems to want to give his parasites the full star treatment, when surely the same observations could be made of other microbial pathogens.

These are minor gripes, however. Zimmer is a fine science writer and if you’re interested in science you can’t go wrong by picking up one of his books. A word to the wise, though: If you’re squeamish, some of the descriptions of parasite biology in this book might leave you a little green around the gills.