The Conqueror Germ

I had a cold. The doctor came

and five assistants, too.

They laid ten icy hands on me,

and now I’ve got the flu.

Martial (90 AD)

Lionel Casson translation

While pursuing a Bachelor’s in history, I grew accustomed to considering the world in the ways usually presented by historians: conflicts of peoples, ideas, and individuals against a backdrop of cultural evolution. There is much to be said for looking at things this way, but a few years after graduation I plucked Plagues and Peoples by William H. McNeill off a Barnes & Noble shelf. McNeill reminded me to try to think outside the box now and then as well.  

McNeill argues that the most overlooked players in historical events are no larger than a bacillus and sometimes as small as a virus. It should be remembered that, prior to the 19^th century when the medical profession first began to save more people than it killed, plague meant an absolute die-back of the population, sometimes on a vast scale. The Black Death was so enormous in its effects in Europe that all general histories cover it, yet this was just one major disease event out of countless ones; the effects of the others often were just as profound. As an example, how would the world be different if Athens had won the Peloponnesian War, as it likely would have but for the plague (still unidentified despite Thucydides’ detailed description) that afflicted the city and army? McNeil, while acknowledging the barbarian pressures on Rome in the 4th and 5th centuries, points out that pressures had been just as severe in the past; plagues which decimated Roman cities (and therefore the tax base to support the army) made the difference this time, and led to the fall of the Empire in the west. In Asia, the interaction of China with its northern barbarians was affected by plagues in way parallel to Rome. In the century after the Spanish Conquest, the native population of Mexico collapsed from some 15,000,000 to 1,000,000, not because of Spanish depredations (which were nasty enough, but no worse than those of the preceding Aztecs), but because the locals had no resistance to European diseases; even mild childhood diseases such as chicken pox were lethal to these inexperienced populations. Napoleon was defeated not by the Russian winter but by typhus, which killed tens of thousands of his soldiers per month – 10,000 in the single week before he entered Moscow.

Prior to modern medicine, human populations adapted to diseases in precisely the same way as other animal populations do. Rabbits in Australia provide a good recent example of animal infection and recovery. Myxomatosis is a fairly mild disease among all species of cottontail rabbits in the Americas, but it is deadly to the common European rabbit (Oryctolagus cuniculus). In 1950 myxomatosis was deliberately introduced to wild European rabbits in Australia in an attempt to control this invasive species. The rabbit population collapsed from 600,000,000 to 100,000,000. The survivors, of course, were those with better resistance to this particular disease. Today in Australia, a much more disease-resistant rabbit population is back up to around 250,000,000.

McNeill was not the first or last to write on the subject, but his book is still a good general introduction.

According to an article in Science Daily, disease may be a hugely underestimated factor in prehistory as well. Studies of genetics and mutation rates allow scientists to estimate the age of some genes and also to estimate the size of primordial populations. It appears that about 100,000 years ago the anatomically modern human population, still restricted to Africa, was reduced to 10,000, most likely due to disease. Most diseases deadliest to humans, it should be noted, emerge from the Old World tropics where pathogens have coevolved with primates for millions of years. Those human survivors had (as we still have) two inactive genes that are active in other primates; the inactivity confers resistance to a wide range of infections that exploit those genes in our relatives. This amplified the immune-response advantage modern humans already had when they later encountered less disease-experienced Neanderthal, Denisovan, and (possibly) Homo erectus populations in Europe and Asia. The newcomer diseases would have been devastating to the locals.

Anthropologists long have argued over whether the extinction of all hominins besides modern humans was a matter of love or war, though there surely was a little of both. For the most part, did the others assimilate with modern humans or were they exterminated? Perhaps neither. Maybe they just got sick. If so, the little bugs that nearly killed off our species then gave us the world. Of course, the possibility cannot be ruled out that new ones may one day take it back.