When Craig Venter, one of the mappers of the human genome, set out on a sailboat cruise to map DNA in seawater all across the globe, he found that a teaspoon of seawater contained on average 50m viruses. While this doesn’t sound particularly reassuring, the bad news is mitigated by the fact that most of these are phages that infect marine bacteria and have no interest in us.
Viruses are parasites, and like all their parasitic kind, they get a free ride from living organisms. The whole point of multicellular life is to create a cosy environment for cells to live in, and evolution has invented all manner of stowaways that want this comfort and manage to get on board, either outside or sometimes inside the cells themselves. While it is not generally in the best interest of a parasite to kill its host and be forced to find a new home, some come dangerously close. Most diseases in the developing world are connected in some way or another to parasitic infections.
Dino Martins’ book is a fascinating compendium of all the critters, some big, some small, some harmless and some lethal, that lie in wait for unsuspecting warm-blooded creatures. It unfolds along four separate writerly tracks. The first consists of lyrical descriptions of nature itself written by a keen observer in love with what he sees. The lyricism extends to scenes of horror: wonderful prose, for example, comes from a description of a rotting elephant carcass in Kenyan noon sunlight, slowly dissolved by a mass of maggots. “A boiling cauldron of maggot stew undulates in steamy waves,” and Martins gleefully cups his hands in the liquefying flesh to sample maggot species while drily noting that the air “shimmers and stinks”. The tone then switches from the lyrical to the taxonomic, listing genera and orders of the various beasties involved. Martins marvels, and so does the reader, at the creativity of nature.
Then comes the charge sheet. One of the most horrifying sections deals with the life cycle of eye worms. I had never heard of them and cannot unread the book. They live in the eye socket – eyelids, conjunctiva, tear glands. Female worms lay eggs that hatch, and the larvae swim out into the tears. Flies drawn to that “weeping of worms” lap up the larvae; inside the fly, the larvae burrow from the gut into the testes or egg follicles, mature, then migrate up to the fly’s head and wait. When that fly visits another animal’s eyes for a drink, out pop the larvae and the cycle starts over.
Understandably, at several points in the book Martins, perhaps reluctantly, switches to “exterminate the brutes” mode after explaining the fiendish cleverness, indeed beauty, of the life cycle of some organism that still causes constant misery or vile, debilitating illness in millions of people. Finally, just when you think you cannot take one more word about any creature other than the domestic cat, he delights the reader with innocently charming anecdotes of his field trips in Kenya and conversations with students and farmers.
I once worked in a marine station at a time when zoology was diversifying into molecular biology and ecology, and remember being concerned that the zoologist itself was among the endangered species. I was wrong to worry: the zoologist has simply adapted, and must now be the master of half a dozen trades to make sense of what nature throws his or her way. In Martins’ multidisciplinary telling, parasites elicit in the reader the sort of feeling one reserves for clever, creative criminals: admiration for the brilliance of the scam, revulsion at the details of the sting, resolve to put an end to it. But in the end, perhaps the most persistent feeling, and one that Martins conveys beautifully, is one of awe at the sheer diversity and inventiveness of the living world.
• Hidden Creatures: Luscious Leeches, Bashful Botflies and the Wondrous, History-Shaping World of Parasites by Dino Martins is published by William Collins (£22). To support the Guardian buy a copy at guardianbookshop.com. Delivery charges may apply.
