As usual, Zimmer tells the story of his interactions with the scientists who study parasites so eloquently and so intimately that it demands your attention and your affection. He makes the often bizarre and macabre-seeming world of tapeworms, hookworms, flukes and ticks exciting and wonderful.
I don't want to spoil too much of the fantastic science in this book but something Zimmer continues to come back to throughout is how much our still-burgeoning understanding of parasitology has begun to really change our understanding of what we thought were hard and fast ecological concepts. One example he gives has to do with the common notion that part of the ecological services that large predators (such as wolves) provide is to reduce the spread of disease through an ecosystem. Prior thinking goes like this: animals that are sick are easier to catch so predators spend less energy if they target these sick animals as food sources instead of healthy animals. This will benefit the ecology because that sick animal is now out of the ecology, it can no longer spread its disease to other members of its species or possibly other species.
The problem is that parasites (and parasites here are defined as all eukaryotes, so animals plants and fungi, that spend all or part of their life cycle either inside a host or feeding directly from a host) often spend part of their lives in one species and another part of their lives inside another species. Let's say there's a species of worm that infects a moose in one part of its live and a wolf in another. Natural selection will have provided that worm with an ability to modify the moose's behavior such that it is easily caught by wolves. It is in that worm's best evolutionary interest to continue its life cycle. So the wolf isn't reducing the spread of disease at all. It's actually actively taking part in spreading the disease. While the previous thinking still can apply to bacterial and viral infections it may have the exact reverse process with eukaryotic parasites.
He also gives descriptions of how many parasites change the behavior of their initial host so drastically that they make food more available for the predators. There is a particular parasite that infects killifish, a small salt marsh fish, and then herons. The behavior of infected killifish is so drastic that it makes them 30% more likely to be caught. This may not be entirely sound but one way to look at this is that food is 30% more available for the herons therefore the marsh can support a larger population of the birds than it could if this particular parasite did not exist. The parasite isn't exactly increasing the amount of food but it is increasing its availability.
This book really made me rethink some of what I know about ecology in some pretty interesting ways. As I mentioned it is written in Zimmer's usual smooth and easy to read prose. His writing is easily accessible and he is one of the best authors for those who are interested in science but feel intimidated or don't think they have a solid background. Parasite Rex was absolutely a great read.