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Review: Carl Zimmer, PARASITE REX

According to one reviewer, "Bacteria and viruses have received all the press when it comes to plagues. In this vividly written book, Carl Zimmer explores the complex world of worms, protozoa and other terrifying creatures that pose an equally great public health threat around the world. These organisms are even more subtle and challenging enemies, and Zimmer provides an excellent introduction to them." -- Christopher Wills, author of YELLOW FEVER, BLACK GODDESS. That is no less than the truth.

Parasites transform lives and alter the course of evolution. But thinking of them as enemies, whether they live on or in us, or in or on other creatures, is too simplistic and homocentric. Just about every type of creature has parasites -- even bacteria have them, in the form of viruses -- and their presence in and on the bodies of other creatures may not do damage to their hosts. One creature's parasite may be another's symbiont or commensal, i.e., one enjoying a mutually beneficial relationship with its host, a relationship that may mutate into damaging parasitism due to many factors such as introduction of yet another type of microbe to the body, a bad diet, etc.

Even so, parasites can and often do horrendous damage to their hosts, human and otherwise. There is, for example, the tongueworm, a creature living on fish that enters a fish's mouth, clamps down on the tongue so the fish can't eject it, gradually eats the entire tongue, then takes the tongue's place in the fish's mouth so that it gets first crack at whatever the fish eats.

Then there are Sacculina carcini barnacles, which invades crabs and fills the entire crab's body with a network of roots.It forms a sac full of its own larvae where the crab's own egg pouch should be, and forces the crab to care for the barnacle's offspring.

Snails can also be rather horrifically victimized when they are infected with the Leucochloridium paradoxum. The parasite's final hosts are birds. To get the birds' attention, the parasite climbs into the snail's transparent tentacles. The striped flukes, which can be seen through the surfaces of the tentacles, look like caterpillars, tasty treats for birds.

And there is the tapeworm. Normally, tapeworms first mature in a cyst in intermediate hosts such as cows or pigs before moving on to humans. But if their eggs should end up in a human body, they will go ahead and form a cyst anyway, often in the brain, where they can eat huge holes in the organ, impacting perception, behavior, and the autonomic processes of the body. If this goes on long enough, the patient dies in a long drawn-out process that can be agonizing.

The examples of the nasty impacts parasites have on their hosts are often enough to turn the stomach of those just reading about them, let alone suffering them. There are countless species of parasites, too. Yet parasites have important and often beneficial niches in the ecosystems of which they and their hosts are part. "Beneficial" is not a word that people infected by various parasites would use to describe their parasites' impacts on their lives and well-being, but in the end, as the Black Death did Europe in the Middle Ages, they may do service to entire species, preventing rampant human overpopulation and ensuring that our descendants will have much better lives than would have been possible if those parasites did not exist. In Chapter 8, "How to Live in a Parasitic World," Carl Zimmer discusses ways in which parasites on invasive species can curb the growth of the populations of the latter and thereby get the damage from invasive species down to a low roar and then no roar at all. And in the end, they may reduce the threat the ultimate invasive species, Homo sapiens, presents to our living world, benefiting us as well as other creatures by bringing our numbers down to something Earth's biosphere can live with without making us extinct or destroying civilization. I'd say that's a plus right there.

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