Learning to Love the Parasite(s) Within
If what you know about disease comes from the television, it must be a daily shock to awaken and find yourself in good working order. HN51, the latest scary avian flu, hasn’t yet become transmissible from person to person; Ebola hasn’t depopulated Boston yet; flesh-eating bacteria haven’t yet ravaged your legs: Somehow, the parade of horrific, though perversely media-friendly diseases has not yet made it to your room. What has saved you (so far!) is probably the raft of antimicrobial products you’ve bought, from your underwear to your dishwashing liquid, right? Or perhaps you subscribe to an unusually bleak version of the Gaia hypothesis, and welcome the prospect of epidemic disease as a necessary corrective to poverty, climate change, and a variety of other human-enhanced ills. These disparate views nonetheless share a belief that disease is antithetical to life, and especially to modern life. That belief makes a kind of sense—almost by definition, disease disrupts an organism’s function. Either we successfully fight it off, or it takes us out.
It is precisely this sort of thinking that Marlene Zuk’s splendid and witty new book, Riddled with Life aims to defuse. Right at the beginning, she proposes a new metaphor for disease. Rather than thinking of ourselves as in a battle with germs and disease, we ought to think of diseases and parasites as “members of our family”: “We do not choose to have them, but our lives are unimaginable without them, and for better or worse, they have made us who we are. Some people have better luck, or less dysfunctional families, and some have worse, but no one grows up without their ancestors’ influence”. Disease, she makes clear, is not an aberration from “normal life”; it simply is the norm, and the notion of a disease-free multicellular organism is a fantasy. As she points out, males in general owe their existence to disease: The great advantage of sexual reproduction over asexual reproduction is that it improves resistance to disease. (Make sure your creationist friends read the book: Zuk reports on experiments involving sexually- and asexually-reproducing animals and their resistance to parasites and disease—it’s evolution in action!) This is a thought-provoking argument—as she points out, at the very least it should give us pause as we contemplate in utero “fixes” for genetic diseases.
While Riddled with Life won’t make you enjoy your spring cold, it may well bring a fresh understanding of the centrality of disease to health, and almost certainly will drive home the diversity and ingenuity of parasites. After a brief recap of Darwin’s relevance to discussions of hygiene, Zuk explains the pyrrhic nature of our drive to eliminate microbes and worms from our environment, followed by an almost mandatory discussion of vaccination. Sex is at the heart of the book—perhaps unsurprising, since her previous book was Sexual Selections: What We Can and Can’t Learn about Sex from Animals, and since her own research focuses on “sexual selection and the effects of parasites on mate choice and the evolution of secondary sexual characteristics.” While the focus on sex isn’t surprising, her examples are absorbing and well-chosen. She concludes with a more sober chapter, assessing the realities and the evident hysteria over emergent disease. Overall, her argument about disease seems to be that if we stand down a bit in our war on germs, we may well enhance our health and that of our children.
Zuk has an amazing gift for turning experiments and facts into stories. The chapter on sexually transmitted diseases is perhaps my favorite in the book. I learned, for example, that while STDs in mammals are usually microbial, in insects, they tend to be transmitted by mites and worms—in part because insect sex is a feat of mind-boggling endurance: “Stink bugs mate for over 10 hours, the more gracefully-named golden egg bugs copulate for up to two days, and the apparent champion, a type of stick insect, has been clocked at 79 days. Even the lowly flea manages half an hour or so, and what is more, can mate with its nether regions while its mouthparts remain attached to its host, an admirable example of multitasking. The achievement becomes even more impressive when one considers the relatively short lifespan of most insects”. But the best thing I learned was about an experiment performed by Anders Pape Møller. Apparently, there is a type of fungus that attacks houseflies by “invad[ing] their abdomens and expand[ing] to fill the body cavity.” When the fly dies, it stays wherever it is, “glued ... by the strands of the emerging fungus,” with an abdomen distended, swollen with a heavy load of fungus. As it happens, female flies with swollen abdomens are ready to have their eggs fertilized. Enter Møller, who gave “flies an opportunity to approach dead flies with and without the infection. Male flies were much more attracted to dead flies with the fungus than to uninfected corpses, and once attracted they often tried to mate with the body, exposing themselves to the parasite”.
So, in short, Møller set up an experiment to test the necrophiliac propensities of flies. What’s especially hilarious about this is that the flies were “much more attracted” to the fungus-filled flies than to “uninfected corpses.” It’s not that they were unattracted to the uninfected corpses ... they just preferred the infected ones. The book is full of experiments of a similarly lively nature.
Recent years have featured a spate of books about the superpathogens that will, the story goes, devastate humanity. And, to be fair, that’s always possible. But Zuk suggests that a healthier, less paranoid attitude toward disease is likelier to help us survive such a catastrophe. Zuk’s is not the first book in 2007 to claim a evolutionary benefit to disease: Sharon Moalem’s Survival of the Sickest makes similar arguments as well. Of the two, Riddled with Life strikes me as far superior, in part because Zuk doesn’t insist loudly on herself as a revolutionary. She is urging the public to take a new look at disease, though it’s one that’s well supported by the research. There are moments where she’s speculating ahead of the science a bit, but those moments are clearly marked. Riddled with Life will change the way you look at public and private health.