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1 April 2008 Appreciating Parasites
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Wine appreciation courses teach a deeper understanding of wine. Marlene Zuk says her book Riddles with Life is an infectious disease appreciation course. It is, but I think it does more than just generate a deeper understanding of infectious diseases. Her evolutionary view of infectious disease offers us a deeper understanding of ourselves.

As with the theory of natural selection, the argument is obvious in retrospect, but the consequences have yet to be fully worked out. First, infectious diseases are not an aberration, a fall from clean, hygienic perfection. Being infected is natural: almost germ-free health and hygiene is a rich-world oddity made possible only by modern sanitation and, to a lesser extent, modern medicine. Second, infectious diseases cannot be banished. They always have and they always will evolve around our control measures and exploit the new niches our changing proclivities open up. Third, infectious diseases can do amazing things, and their hosts—us included—do amazing things in response. For instance, infectious diseases can make animal hosts more secretory (think mucus, vomit, feces), more mobile, obvious, angry, brave, hungry, long-lived, paintolerant, sterile, larger. In turn, animals respond to infections by having more sex, or becoming monogamous, committing suicide, engaging in self medication, developing germ phobias, rolling in feces of other species, eating spicy food, having acne, eating dirt, living in groups, living alone, engaging in herbal medicine. Maybe the evidence for some of these responses is a little flaky, but you have to include the gee-whiz stuff in a pop science book, and given the sheer volume of such phenomena, it's hard not to agree that host-parasite interactions are powerful forces.

If you really need a male, why not mate with the first one you find? Why do you need new, different, or particular genes in your offspring?

Infectious disease is thus a pervasive force in historical and contemporary life. Zuk lays out a number—a good number—of reasons why that is important. My bet is that she has not got them all. But if you have not read any evolutionary medicine, and you think, for instance, that all disease symptoms are bad, that parasites eventually evolve to be nice, or that failures in our genomes are a sufficient explanation of common “genetic” disorders, Zuk's book is as good a place as any to start. Indeed, her style is lighter and more fun than the pop-Darwinian medicine competition (cats have “bad fur days,” hygienic nest builders are “avian Martha Stewarts”), and she revels in wordplay and punchy sound bites even when they are not her own (from Norman Stoll: humans have worms because of man's “ineffective insulation from his own excretory products”). Unlike other popular science books written by practicing scientists, this one is not an excruciating read. As you romp though the book, you can't help but think Zuk enjoyed writing the thing.

There are also other popular accounts of the evolutionary conundrums posed by sexual reproduction and mate choice. Why throw away half your genetic representation in the next generation by letting a male fertilize your eggs? If you really need a male, why not mate with the first one you find? Why do you need new, different, or particular genes in your offspring? The answer, favored by Zuk and others, is that genetic change is always necessary to keep up with pathogens, whose evolution makes resistance inevitably transient. But unlike the pop-science competition, Zuk draws a rather profound conclusion from this. If mate choice is about infections, then a very large component of the human condition evolved in response to disease pressure. Moreover, we are all different because our parents had sex. That I am different from you is a legacy of infections past. Celebrate human diversity. It exists because of the bugs.

Celebrate human diversity. It exists because of the bugs.

But the most interesting implication of the evolutionary perspective is a tantalizing view of the future of neuroscience. The argument is analogous to the hygiene hypothesis, one of the hot topics in immunology. Asthma, allergy, and other autoimmune problems became much more common in the 20th century, at least in the rich world. A growing body of evidence supports the view that de-worming was responsible. Parasitic worms secrete substances to dampen our immune response. The hygiene hypothesis posits that our immune systems evolved to cope with this down regulation. Remove that down regulation and you have excessively aggressive immune reactions. Increasingly, it looks as though there is something in this idea. Give worms to people with autoimmune “dysfunction,” and they get better.

Which, as Zuk points out, raises a very interesting question: which other organ systems evolved to deal with infections? Surely all of them. Which of these are misfiring as we make people “healthier”? And here's the shocking notion: Throughout the animal kingdom, infections are capable of manipulating host behavior to boost their own transmission, no question of that. It would be extraordinarily odd if parasitic infections were not continually probing human sensory and decisionmaking systems in an ongoing attempt to manipulate our behavior to enhance their transmission. They may even be constantly failing—maybe our brains are more robust than our immune systems—but they must be probing. How much of our neural complexity is a necessary defense against manipulative invaders? How much of the enormous redundancy is to provide system-level redundancy against attack? How much of the complex process of wiring a brain during development is actually to prevent covert attack? If our brains have evolved to deal with a continual barrage of manipulative intruders, what happens when that attack is removed by hygiene and modern medicine? Can the mind of a healthy body be healthy?

Andrew Read "Appreciating Parasites," BioScience 58(4), (1 April 2008). https://doi.org/10.1641/B580416
Published: 1 April 2008
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