This is an oldie but a goodie.
Parasite Rex is a broad overview, not just of its titular subject, but also of the ways that the notion of the parasite is evolving, and how it should be understood by people. It traces the genealogy of the study of parasitism from the origins of the word in Greek all the way to the 19th century disdain of such creatures as degenerate life forms and evolutionary dead ends that depend utterly upon their host and gave rise to the moral stigma associated with being called a parasite.
Zimmer’s project is to demonstrate just how complex and well-adapted parasites are as lifeforms, and the ways in which they play an important part in balancing their various ecosystems. Of course, the term parasite is a broad one; it can range from anything from a single celled organism to a large animal like a cuckoo that lays its eggs in the nests of other birds.
Parasites are, to put it simply, organisms that are adapted to survive and reproduce at the partial expense of other creatures. Their playground is not the savannahs, hills and jungles of your typical animal, but the bodies of other organisms. In that respect they are as beautifully adapted to these environments as the gazelle is to the Serengeti.
The bodies of organisms are treacherous places that have evolved immune responses to foreign invaders. In that respect, parasites are resourceful creatures that constantly innovate (via evolutionary adaptations) creative responses to thrive amidst it, ranging from hiding in plain sight within red blood cells, or constantly changing forms so that T-cells can’t keep track of them.
Having infested their hapless hosts, parasites need to ensure they are able to propagate to the next stage of their life cycles. And it is in this where parasites get much of their well-deserved reputation as nature’s horrors – employing all sorts of creative but ghoulish ways to make their hosts serve their ends – from Sacculina turning male crabs into hapless castrated zombies, to toxoplasma gondii making rats more reckless to ensure they are eaten by cats, hence allowing the parasite to settle into its true feline home. Sometimes these adaptations allow the host to live relatively normal lives, albeit as unwitting Typhoid Marys; in other cases, the death of the host is the prerequisite of the flowering of the parasite into its succeeding life stage.
But despite these macabre innovations, Carl Zimmer gives parasites their due as vital players in the diversity of life. Parasites may have played a role in accelerating the evolution of sexual reproduction, energy intensive as it might be, as a means of allowing animals to generate enough species diversity to throw off the pernicious effects of parasites.
And parasites play a bigger role in ecosystems than might first appear. Consider the relationship between a predator and prey. The usual narrative is that predators keep populations in check and increase the health of the herd, because usually the weakest prey is the meal of the predator. But perhaps this salutary vision is just an illusion, and it is the parasite that weakens the host to allow it to be captured by the predator, which is its next presumptive host. Parasites, though ghoulish at times, may be the signifiers of healthy, robust ecosystems.
Even human evolution has been shaped in large part because of the existence of parasites. We’ve evolved various responses to common parasites, so much so that in our modern industrialised society, the eradication of such parasites has indirectly led to the rise in new autoimmune diseases such as colitis, as well as an increase in allergies.
Zimmer writes in a dense but approachable style, peppering the narrative with interesting stories and examples of parasites and their behaviors, painting an erudite picture of how we should think about the broader role that parasites play in the web of life. Written in the early 2000s as it is, perhaps some of the facts and hypotheses described have been validated or disproven, but ultimately, Parasite Rex still remains a comprehensive and engaging starter guide on how we should think about parasites.
I give this: 4 out of 5 guinea worms