Disease and Cold Drive History

 Update:  For example, drought.

The effect of disease on historical events has usually only been mentioned in extreme cases, when it is obvious that at least some influence must have occurred.  The effect of smallpox on the Aztecs, or the Black Death on the economics of Europe receive some mention, but even in those cases summary histories can leave them out. Amazing, but true. There has been some increase over the last twenty years of historians addressing the issue of disease directly, and the last five years has seen an explosive growth in that approach because of what we can learn from archaeology rather than written records.  What has then happened is that the text historians have doubled back and acknowledged "yes, this was there all along, but because we could not clearly understand symptoms nor measure extent we could not make definitive statements." So they mostly said nothing.

Or, as I suspect, they preferred other explanations, as we all have, due to the training in how to look at history we have all grown up with. I may be partly guilty myself.  I have heard doctors offer up possible medical explanations they have run across in their reading, only to be greeted with blank looks and polite smiles.  No, silly.  It's kings and battles and trade routes and technological advances. Diseases are just there all the time and have only a marginal effect. I have been only slightly more sympathetic until the last decade,

I recall reading in the 1970's an article on the colonisation of New England which claimed the Puritans were remarkably fortunate in when they arrived, as the Indian population was only about a third of what it had been in 1607.  The idea was that they had wiped each other out in a series of battles. Otherwise, the Englishmen would not have had a chance. We now know that those natives were first depleted by the diseases they had picked up in trade contact with Europeans in what are now Maine and Nova Scotia.  The battles came as a result of that, as tribes tried to move into territory devastated by disease. They died of European diseases, then wiped each other out before 1620. That remaining third was further depleted by additional diseases after year-round settlers came.  In temperate climates the losses were fewer.  In New England, it may have been "only" 80% of the natives who died from disease.  In Mesoamerica, with denser settlement, it may have been 95%. In the absence of any understanding of germ theory, this was doubly devastating psychologically.  Your leaders, your shamans, your customs had all failed. There was nothing to cling to.  You concluded that the invaders were favored by the gods. Who wouldn't?

India and China, both of which have both temperate and tropical zones, have centuries of the same pattern repeating.  An invader arrives from the Northwest.  They are from temperate zones, tall, strong, and healthy.  They easily defeat the inhabitants and take over lands ever southward. Then they catch tropical diseases, they die, and they retreat back northward.  Sometimes they go all the way back to lands they came from.  Other times they go back only halfway, and settle.  They gradually acquire a little immunity to the tropical diseases, but the two groups essentially settle into a north-south division of very different groups.  The Dravidian languages are not at all closely related to the Indo-Aryan languages. Take whatever political explanation you like about weaponry, organisation, and culture. The climate line, and thus the disease line, explains a thousand years of history more exactly. See also Mongols, China. (The Mongols later also destabilised the entire Eurasian continent, by disease as much as horsemanship.)

The new invaders also bring new diseases, which can devastate the tropical population. Yet in the end, holding the territory becomes impossible for those from temperate climates. Only those who have a culture of raiding rather than settled conquest can survive on the edge of those lands: Saxons, Picts, Huns, Mongols, Sueves, Yamnaya, Apaches, Comanches, Bedouins, Vikings, Berbers.* Europeans could extract wealth from the coasts of Africa, but even now have trouble living in the interior. Malaria and other diseases take their toll. If a section of jungle is cleared for cultivation, the mosquitoes take over, and the death rates soar.  In a stable period, the humans and the microparasites establish a balance over time, neither conquering, neither eliminated.


The temperate zones have their own diseases as well, often those which can adapt by going dormant for months in dry or cold seasons. The macroparasites, the other humans, upend the balance, with unpredictable results.

The inhabitants of cities have a similar "advantage" over the outside invaders. They may be sickly and less strong, but diseases have already wiped out many of their siblings in childhood. They have the immunity of those who have been exposed to many more diseases. The invaders arrive again and again, tall and strong, but cannot hold the cities they take. The Goths and Vandals wrought havoc, conquered and settled in the Roman Empire, and ruled sections of it for a short time. Then they were inexplicably defeated by the armies of the empire a few decades later. Their star burned bright and flamed out. Historians have offered many possible explanations about military mistakes, trade routes, feckless emperors. All of those things have some truth. Disease is not 100% determinative. But the new thinking is that if you have to pick one factor to oversimplify with, go with disease, and second, natural disaster.

Justinian, emperor of the eastern empire, was determined to retake and restore the west in the 6th C. He succeeded. He put the Goths and Vandals on the run. Unfortunately, the weather was already turning colder than the warm centuries that marked the height of Roman power when at least two major volcanic eruptions elsewhere turned the entire world cold in the late 530's. Those produced the coldest years in the last 2000. Harvest were bad, and then nonexistent. Onto this was added the Bubonic plague - what was long speculated has recently been confirmed by DNA - which wiped out at least half, and perhaps as much as 75% of the population of the western Mediterranean. The cities collapsed, the various armies of Goths and eastern empire raided the countryside for whatever food they could find. Rome's population fell from 350,000 to 50,000 in a century or less.

When disease and starvation take your population out, they take the old and the young.  In 546 AD in the western ex-empire, there were no young children, no old people in the villages. This was less true as one headed north, but still essentially true all the way up to Scotland and Scandinavia.  There had been people, and cities, and trade, now there was not even food. Cultures with a tradition of raiding survived.  Those with traditions of armies and governments did not.  In the Eastern Empire this was nearly true as well, as that half almost fell a few times in those centuries.

Dense populations create more diseases, and sometimes a new variant can be devastating.

I don't think it would be efficient to try and link you to various episodes of a couple of the podcasts I am listening to nor the authors they recommend, but the original text is William MacNeill's Plagues and Peoples from 1976.  It was updated in 1998 in an effort to explain AIDS to the general reader. His explanations were sniffed at in the 70's, drew grudging respect by the 90's, and are now considered to be the foundation of the art of seeing history through the prism of disease.

Historians who credit climate for dramatic upheavals seldom mention that it is cold that is the problem, not warming.

*The idea that the Huns or Mongols could have conquered Europe except for good luck for my ancestors because the Asian warriors had to double back because of battles of succession back East is almost certainly false.  They conquered all before them, but they were steppe peoples who depended heavily on grasslands for their horses.  The steppe ends in Hungary, and they could only raid beyond that. They were not turned back by Pope or battle.  They raided as far as was profitable and went home. There is also evidence in both cases that their ranks were slowly depleting to new diseases.  They may have brought worse than they got, but their supply lines were longer.