Here's a prospect to make the blood run cold. The Ebola virus emerges out of its African rain-forest lair and begins one of its periodic depredations. Only this time it appears in a mutated form. It spreads itself through the air, like the flu, rather than by direct contact with body fluids. And it retains its 90 percent mortality rate. Even compared with the most notorious microbial human killers, past and present, that would be a plague of unprecedented ferocity.
It would take a relatively simple bit of genetic adaptation on Ebola's part. As Andrew Nikiforuk anthropomorphically puts it in The Fourth Horseman, the virus seems "hell-bent" on finding alternative pathways to blood on which to ride to new victims. To make matters worse, we don't have a clue where in the rain-forest Ebola hides. It appears, kills, and fades away-the perfect guerrilla.
What we do know, however, is that we've uncovered Ebola, and other recently emerged viruses, because humans have moved deeper and more intrusively into the rain-forest than ever before. While this process of penetration, provocation, and emergence involves a virus, it encapsulates Nikiforuk's thesis about the way in which disease-causing microbes-both bacteria and viruses-have reacted to human activity. And the crucial word in that last sentence is "reacted".
We have the most intimate of relationships with bacteria. They are both our ancestors, and our "number one life-support system", Nikiforuk writes. By and large, they nurture and protect life, rather than destroy it. They constitute, in effect, "one big superorganism", which, over the last two and a half billion years, has learnt how to live successfully and to maintain and regulate the planet. The superorganism, generally benign and supportive, kills only with good reason-under provocation by its human relatives-"when people have kicked or trampled its frontiers violating unwritten bacterial codes. No other offspring or dependent species has disturbed and challenged the superorganism as much as we have."
The disordered and chaotic succession of human civilizations has produced agriculture, animal husbandry, large-scale war, too many people, and, more recently, tourism and widespread pollution. These are some of the human activities that rile the superorganism. It responds to these irritants by unleashing its killers, which both thrive in the aberrant conditions we have created, and retaliate for their creation.
Our reaction to the superorganism's reaction is to take to the battlefield, Nikiforuk writes. Twentieth-century science and medicine has adopted the "germ theory": "If one germ equals one disease, then one drug should take care of it." They kill us: we kill them. This is nonsense, Nikiforuk argues-based on an incredibly distorted view of germs.
In fact, humans are "the aggressive architects of plagues". As we create our overbearing civilizations, we can't avoid upsetting the natural balance which the superorganism has helped to create and maintain. The morbid result is what the nineteenth-century father of modern epidemiology, Rudolf Virchow, called "abnormal conditions": changes in human social conditions and health. The superorganism's epidemics and plagues signal these social crises, and also serve as the earth's "first-line defence mechanism" against our insults: crowding, pollution, deforestation.
This is a revision of Nikiforuk's original 1991 book. He has considerably revised his account of AIDS, made minor changes to some of the other chapters, and has added two chapters as well. The latter don't make happy reading. "The Bacterial Renaissance: Undying Germs" sets out the effects of antibiotic misuse, and our frighteningly fast slide back into our accustomed impotence in the face of bacterial infections. "Ebola's Apprentices: Emerging Viruses" describes some of the viral humdingers that are popping up as humans hack and burn their way into hitherto wild rain-forest and savannah. Nikiforuk invites us to consider not the remote possibility, but the imminent probability, that one of these "biological missiles" will zap us hard.
Much of the rest of the book is unchanged from the first edition. It tells the story of past plagues and epidemics-malaria, leprosy, bubonic plague, smallpox, syphilis, the Irish potato blight, tuberculosis, and influenza-some of which have never stopped taking the "fourth horseman's quarter", or are reviving and may do so again.
Although he sometimes comes dangerously close to ascribing single microbial causes to vast and complicated events-the fall of the Roman Empire because of malaria, for example-Nikiforuk reveals, or reminds us of, the huge effects that disease has had on us since the Neolithic revolution forever altered our relationship with the planet. Perhaps the most terrible example was the Amerindian holocaust, when smallpox, helped by plague and tuberculosis, killed maybe a hundred million people in less than a hundred years. This great dying allowed Europeans the illusion that they were settling a more or less empty new world-a wilderness-rather than a recently depopulated continent that had been rich in diverse, complex, numerous, and ingenious societies.
The "black death", Nikiforuk argues, had immense consequences. It helped destroy the traditional feudal social structure; it encouraged the weakening of centralized spiritual authority, and spurred on the Reformation. Bubonic plague drove European merchant-adventurers, short of customers (eventually, the plague almost halved the population of Europe), to look for markets in Asia, Africa, and the Americas-the beginning of what would be labelled imperialism. And the centuries-long harvest of lives by the plague changed the way people thought about Nature. "Once revered or at least respected, Nature was now an enemy to be feared and tamed." Ever since, our engineers, scientists, and doctors have devoted themselves to the subjugation of Nature, including, it must be said, the germs that continue to kill us.
AIDS provides Nikiforuk with a contemporary demonstration of the superorganism's response to human perturbation. The disease can't be described as merely the terrible consequence of one virus. It is instead a creature of scarcity and deprivation in Africa; the result of a cocktail of sex, drugs, and blood in North America. In both cases, sexual promiscuity helps to compromise faltering immune systems, and AIDS enters and kills. The virus is yet another sign of "genuine disturbance in human culture". Humans have yet to learn, Nikiforuk concludes, that they must take responsibility for the biological consequences of their actions.
This is a fascinating and frightening book. And while Nikiforuk concludes with the upbeat reminder that we can't ignore the First Horseman, Hope, that seems remote comfort, at least in the near future. Our species appears to be so irremediably committed to the creation of "abnormal conditions"-indeed they're the inevitable by-product of most human activity-that it will take us a long time to learn, if we ever can, how to coexist equably with the superorganism.
In that case, we must presumably resign ourselves to continuing microbial culls, like the heedless and foolish animals we are. 

Derek Lundy is a Toronto writer and lawyer.