The epidemiological zoo

Arno Karlen's book is a well-written survey of the grim intertwined histories of our species and of the diseases that plague us. It ranges widely and there are few illnesses that are not dealt with at least briefly. Nobody who reads it can help but come away concerned and depressed by this account of disasters that show no sign of abating.

But despite the innumerable diseases chronicled in its pages, his book tells only part of the story. We do not know, for example, how many plagues might have spread through the populations of our neolithic and paleolithic ancestors, or how often our earlier progenitors Homo erectus might have been beaten back by disease as they began their spread from Africa to the rest of the Old World. The plague of Athens that began in 430 bc, chronicled by Thucydides, is the first plague of which we have a written record, and we still have no idea what the disease itself might have been. But there are likely to have been many more plagues in the long ages before recorded history, testing the ability of our ancestors to survive and shaping the genes of our immune system.

Karlen is careful to give a balanced picture of plagues' recent history. One of the most horrifying parts of the book is his account of the great plagues that swept through the native peoples of the New World after the arrival of the Europeans. The plagues actually outraced the conquerors - Captain George Vancouver, who made the first contact with many of the prosperous tribes of the Pacific northwest, found bleached bones and skulls littering the beaches he visited. These were the grim result of the germs that had crossed the continent to meet him. And yet, as Karlen points out, the native populations of North and South America had been anything but healthy. He recounts the decline of the Maya empire, its peoples weakened long before the arrival of the Spaniards by disease and malnutrition. The healthy, happy native living in a germ-free state of nature is certainly a myth.

So many diseases have afflicted our species that the book is able to give only glancing attention to most. This is part of its strength, but it is also a weakness, because the reader gains the impression by the end that all these diseases are equally important. For example, in terms of the numbers of people affected, and the ease with which it can be controlled, Legionnaire's disease is dwarfed by the dreadful resurgent spectre of tuberculosis. Yet it receives almost as much attention. The book makes little attempt to examine these diseases' various causes. Some, such as measles, are spread by ignorance and carelessness by the populations affected, and by foolish false economy on the part of governments. Others, such as TB and the bacterial and the viral diarrheal diseases so prevalent in the underdeveloped world, are the direct result of poverty. Still others, such as Aids, certainly had their genesis in societal upheaval and ecological disturbance. Some are hard to cure, while some are relatively easy. Yet all the creatures in this epidemiological zoo are treated by the author with an equal amount of trepidation.

Why so many diseases? As an evolutionist, I cringed at explanations that bordered on the facile. "Nature often seems to correct a severe population crisis with a mass die-off. The most efficient way to do so is by means of an infectious disease I" Diseases certainly do often act as population controls, but this sort of thing conjures up an image of Mother Nature tut-tutting as she applies a spot of discipline - for our own good, of course. Things are a bit more complicated, as well as a good deal less anthropomorphic, than that. And these days, thanks to science, there is no longer an obvious relationship between disease and population growth - at least in our own species.

And what of the future? Karlen say a number of sensible things. In the developed world, we are moving from a period of approximate equilibrium between populations and diseases to a period of uncertainty as we confront resurgent and new pathogens. The amount of disease in the world now is not larger than it was a few decades ago; it is almost certainly smaller. But it is, shall we say, more democratic in its distribution, as a result of immense worldwide migrations. Diseases have begun to breach the defences of the tight little islands of racially and culturally homogeneous peoples in northern Europe and America, peoples who until recently were able to ignore what was happening elsewhere in the world.

Brief treatment is given to the difficulties faced by scientists in coming up with new ways to control disease. When as a child I had a tonsillectomy, I was kept in the hospital for the better part of a week. A child undergoing a similar operation would be out on the street within hours now, partly as a cost-control measure and partly from fear of such new hospital-dwelling monsters as drug-resistant staphylococci. The book would have gained in value if more time had been spent on the problems facing scientists and doctors and on the new and exciting research that these difficulties have spawned. So long as antibiotics killed bacteria easily, there was little incentive to learn about them. Now there is a great deal. Drug-resistant bacteria can swap resistance genes with other, sometimes only distantly related bugs. We have a powerful incentive to find out how this happens and how to overcome its consequences. Luckily scientists are rising to the occasion.

Despite all the troubles recounted in this book, there is no doubt that in humans pathogens have met their most cunning foe.

Christopher Wills is an evolutionary biologist, University of California, San Diego.