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This chapter is from the book

Every cloud has a silver lining: our debt to disease

The way epidemics have intervened in history shows that disease is not just a uniformly negative matter. The outcome of an epidemic may be quite complex, especially over the long term. Whether we regard any particular outcome as "good" or "bad" depends partly whose side we are on and partly on the relative weight we give to short-term versus long-term effects. In this book, I point out the positive effects of epidemics. This is not because disease is beneficial overall, but because these less obvious beneficial side effects often are overlooked. If a virulent plague rages through society, the obvious response is to stop it by whatever means possible, not to sit around fantasizing about its effect on future centuries.

Not surprisingly, we normally think of infectious disease as our enemy. When a successful program of vaccination wipes out a blight such as smallpox, we feel no remorse that a unique life form has suffered extinction. When we learn that throughout the course of human history infectious disease has been responsible for more deaths than war, famine, or any other cause, this only confirms our viewpoint. Indeed, the victories we have achieved over infectious disease are among modern man's greatest triumphs. Today industrialized nations have largely brought infectious disease under control. Unlike our predecessors of only a century or two ago, nowadays we mostly die from heart disease and cancer. Our longer lives give us time to reflect on the other side of this issue, and I argue that, paradoxically, we also owe a great debt to infectious disease.

This approach is not merely idle intellectual self-indulgence. Infections that still threaten us either tend to cause disease in a subtler manner, or else they remain dangerous for other complicated reasons. The classic modern-day example is AIDS. This disease does not actually kill directly. Instead, it damages the immune system, allowing other diseases, impotent by themselves, to gain a foothold. Perhaps it is time for humanity to also take a more indirect and subversive attitude.

Over the long term, a positive side to disease emerges. Granted, if large black swellings are appearing in your armpits and you're about to die of bubonic plague, you'll find it difficult to maintain an unbiased perspective. Nonetheless, although the Black Death epidemics that ravaged Europe in the Middle Ages were devastating at the time, they had beneficial effects on a more global and futuristic scale. They shook up the repressive feudal system and, in the long term, made a major contribution to the evolution of Western democracy.

On the negative side of the balance sheet, we have the millions who died painful deaths in the plague epidemics. On the positive side, we must not forget those other millions who would have died in misery and poverty if industrial democracy had been delayed significantly. In our horrified emotional reaction to epidemics, we normally forget this latter aspect. We do not know for sure how many children would have died in infancy each century if the feudal system had continued. However, if we compare the infant mortality of 30%–50% that prevailed before industrial democracy with the less than 1% infant mortality of today, we can clearly see that millions of innocent lives have indeed been saved.

On a more individual level, Charles Darwin probably caught Chagas's disease while on his famous voyage on the Beagle around South America and the Galapagos Islands. His resulting poor health kept Darwin at home for much of the rest of his life. Instead of wandering off on more expeditions to observe nature and collect specimens, he stayed put and pondered the origins of living things. This may well have played a major role in Darwin's compilation of the most influential book of the last few centuries, The Origin of Species.

As already remarked, whether such indirect effects are "good" or "bad" depends on your perspective. Should we consider the happiness of the individual, the benefit to a particular group, or the overall betterment of mankind? For that matter, how do we define the "betterment" of mankind? Whatever your outlook, the effects of infectious disease have been undeniably important in changing the course of history. Perhaps it is not too fanciful to think of "good" and "bad" diseases. Some diseases, like bubonic plague, may have had some beneficial long-term side effects on human society as a whole. Others, like sleeping sickness, have no positive aspect. Whatever our moral perspective, the effects of an epidemic on the overall fortune of a tribe, nation, or even a whole continent may be quite different than the immediate effects on the victims.

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