Today, we wonder who feasts upon whom. The University of Houston's College of Engineering presents this series about the machines that make our civilization run, and the people whose ingenuity created them.

I've just read the most remarkable book, The Human Condition, by historian William McNeill. McNeill writes human history in 78 pages by telling it in terms of microparasites and macroparasites.

Microparasites are any of the small forms that live off us -- micro-organisms, funguses, insects, or small animals like mice and rats. McNeill suggests the word macroparasites for humans who feed on other humans. It may sound a bit like Marxist theory, but people who produce no material goods are in danger of being called parasites. However, modern societies depend on vast divisions of labor. We cannot function unless many people serve the common good in ways that don't directly produce goods.

Older, more primitive societies had far less division of labor. In those societies you either produced goods for survival or you were a parasite. So McNeill traces the way parasites (large and small) follow, and even shape, the formation of civilization.

We really opened ourselves to serious attacks of microparasites when we gave up hunting and gathering and took up agriculture. As we formed more population-dense, and static, settlements, we made better targets for epidemic disease and infestations. Planted crops are more vulnerable to fungus or insect attack than wild ones. Diseases can't prosper in a human population until it's large enough to let them move from hosts who've been rendered immune into fresh hosts who have not. That's why chicken pox and mumps are so often children's diseases. For millennia, epidemic diseases served as agents of population control. Now we're just trying to learn to control population without that kind of help.

Forming into communities led to the creation of surplus goods, and that left us open to macroparasites -- to thieves, then war-lords who levied taxes. But thieves mutated into tradesmen who served farmers by moving goods in ways that served their interests. Taxation mutated into more-nearly-agreed-upon means for fulfilling needs of the people who produced goods.

As trade routes opened up, a commerce in microparasites attached itself to trade as well. The bubonic plague, for example, entered Europe on a trading ship. At the same time trade also created new macroparasites -- pirates and highwaymen. Those parasites have also mutated into defensive navies and police -- into warriors who try to serve the common good. Those warriors in turn often mutate into macroparasites on a global scale -- warring armies become new agents of theft, out to steal whole nations.

Reducing history to waves of parasites may seem strike you as gloomy. But the view carries an odd ray of optimism. For each macroparasite mutates. The huckster mutates into the technologist who reduces the labor needed to produce goods. The witch-doctor becomes the physician or the scientist. And hope remains alive.

I'm John Lienhard, at the University of Houston, where we're interested in the way inventive minds work.

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McNeill, W. H., The Human Condition: An Ecological and Historical View. Princeton, NJ: Princeton University Press, 1979.

I am grateful to Tom McConn, UH History Department, for providing the source and suggesting the topic.