Humanity is fortunate that solving problems often does not require understanding how or why our solutions work. Being close is good enough. We used aspirin for centuries before medical science revealed its mode of action; the steam engine preceded the physicists' understanding of vacuum pressure by years; and bronze was invented centuries before chemists and metallurgists could explain its properties. Likewise, for thousands of years, weapons of war were lethal enigmas. The ballistic equations describing the trajectory of a spear or arrow, the mechanical principles underlying a catapult, and the chemistry of gunpowder were mysteries, but scientific ignorance did not preclude using and refining these weapons. And no form of killing more clearly exemplifies our capacity to exploit natural processes beyond the limits of our knowledge than does entomological warfare.
Humans have long understood that disease could be passed among indi-viduals.1 A clay tablet from the 17th century bce suggests awareness that sick people were contagious and that quarantine could prevent the spread of disease. The ability of insects to transmit pathogens was vaguely suspected by the ancients. Various records allude to flies as having a magical role in illness. And for millennia, the cultures of the Mediterranean region drew connections among mosquitoes, marshes, and malaria. But they erred in concluding that the illness was caused by breathing the swamp vapors; hence, the term mal ("bad") aria ("air") for the disease.
Despite the biological blunder as to the origin of malaria, ancient civilizations made impressive progress in controlling the disease, and the work of Empedocles is a prime example.2 This controversial character from the fifth century bce was a gifted philosopher-poet-physician and something of a charlatan, claiming to revive the dead and control the weather. His powers over nature extended to eliminating the fevers that plagued his region of Sicily. The shady but shrewd Empedocles oversaw the draining of nearby swamps, and along with the bad air went the pesky mosquitoes, which were never associated with the success of the remedy.
Once people figured out that draining marshlands could diminish disease, military strategists latched onto the possibility of reversing public health. Forcing an opponent to occupy noxious habitats could yield victory. Fevers could be deadlier than swords. Not 50 years after Empedocles saved his region from malaria, the Athenian invaders of Sicily were decimated by this disease during their siege of Syracuse.3 Scholars debate whether the Sicilians maneuvered their defensive line to force the Athenians to establish summer camps amid the marshes or drew their enemy into the wetlands through the ruse of negotiating surrender. In either case, mosquitoes and malaria conspired to break the siege. A century later, the Carthaginian army was wiped out by malaria in the same lethal landscape that had defeated the Athenians, demonstrating why military commanders would be well advised to study history.
The Macedonian strategist Polyaenus provided a detailed account of a particularly villainous, passive-aggressive use of insects.4 In 306 bce, a despot named Clearchus seized control of Heraclea, on the Black Sea. Despite being a student of Plato, Clearchus came away with an affinity for tyranny rather than philosophy. He surrounded himself with a circle of brutal henchmen who robbed and raped the locals. When the people appealed to their leader to control his lawless lackeys, Clearchus merely shrugged and offered the ancient version of "boys will be boys." The only recourse that he could offer was for the citizens to build him a walled acropolis to restrain his dishonor guard.
The Heracleans complied, but Clearchus's goons continued their felonious ways. Fearing insurrection among his increasingly fractious subjects, Clearchus arrested Heraclea's democratic council on trumped-up charges. This was a fine first step, but he needed to rid himself of dissidents within the general populace. Using his own men to conduct a political cleansing would surely feed the flames of revolt, so he turned to nature.
Clearchus drafted an army—including his most likely opponents from the citizenry—for a campaign against the city of Astachus. He led the men into western Turkey during the hottest days of summer, but the invasion was a ruse. His purpose was to station his recruits in the marshy lands outside of the city walls. Clearchus and his personal guard established camp on the high ground, leaving the troops to occupy the low-lying areas. From this vantage he had shady trees, fresh breezes—and a fine view down to his condemned army.
Fevers soon swept through the camp, and by the end of the summer his army of potential insurgents was dead or morbidly ill. Clearchus returned to Heraclea, his political victory cleverly masked by a military defeat. His subjects never figured out the depraved scheme of their ruler, and the tyrant never knew that he owed his reign to a legion of disease-carrying insects.
While maneuvering opposing forces—or in the twisted case of Clearchus, one's own troops—into contact with mysteriously but predictably unhealthy landscapes was a viable strategy, this approach to entomological warfare only worked if suitably soggy conditions were available. However, even without biologically hostile environments, insect vectors could be conscripted by savvy commanders without knowing the details of epidemiology. An imprecise concept of contagion was sufficient to deploy entomological weaponry.
The earliest use of germ warfare might have involved history's most infamous pathogen, Yersiniapestis, and the world's most legendary booty, the Ark of the Covenant. The historian Adrienne Mayor has suggested that this sacred chest might have been guarded by plague-infected fleas.5 The story begins with the Philistines' capturing the Ark from the Israelites at the battle of Ebenezer in 1050 bce. Fearing that Yahweh would be angry with them for trouncing his chosen people, the Philistines hid the booty in their capital, Ashdod. But it's hard to fool omniscient deities: "After they had moved it [the Ark], the Lord's hand was against that city, throwing it into a great panic. He afflicted the people of the city, both young and old, with an outbreak of tumors in the groin" (1 Samuel 5:9). These swellings were probably buboes: bacteria-laden lymph nodes that are a classic symptom of bubonic plague, or the Black Death. Some biblical interpretations refer to these swellings as "hemorrhoids," which is almost certainly a mistranslation and raises the question of whether an outbreak of inflamed rectal tissue would be a suitable divine punishment.
The citizens of Ashdod desperately wanted to distance themselves from the Ark and the associated plague. So they passed the chest on to Gath, where the people also came down with the horrible swellings—and they passed it on to Ekron, where the disease struck again. Seeing the pestilential pattern, the Philistine priests ordered that the Ark be sent back to the Israelites, along with a load of golden statuary to appease Yahweh. With the return of the Ark, the plagues ended and the Philistines learned not to mess with the Israelites and their god. But what had happened in epidemiological terms—was there an entomological booby trap hidden within the Ark?
The Philistines who brought the Ark home from battle might have brought the plague with them, and those who escorted the chest from city to city could have spread the disease. Mayor proposes another explanation. The Philistines knew that peeking inside the Ark was absolutely forbidden, but perhaps the temptation was irresistible. So Mayor wonders, "Does the story of the Ark suggest that the chest might have contained some object, such as cloth, that harbored aerosolized plague germs, or an insect vector that infected the rodents in Philistine territory?" The microbe that causes bubonic plague is transmitted to humans by infected fleas. While there is a pneumonic form of the disease spread by coughing and sneezing, this variant is not associated with the buboes described in the book of Samuel.
The Israelites who set the pathogenic trap might have surmised that fleas from a sick person would ambush Ark peepers, but such an understanding of insect vectors is improbable. More likely, the Ark's protectors armed the chest with a plague victim's clothing, which was thought to be the source of contagion, while actually harboring stowaway fleas. Bacteria cannot survive for long in the environment, but a flea can live for months—the record being 513 days—between blood meals.6 A few of these insects riding along in the Ark would have made a very hungry and lethal surprise for anyone who tried to access the chest's sacred contents.
An insect-charged booby trap might be viable when protecting a discrete, valuable resource such as a wooden chest, but it is hardly feasible in the course of typical warfare. In normal military conflicts the would-be entomological warrior must have some means of imposing the infected insects on the enemy. But given their lack of scientific knowledge, ancient armies would have had to stumble upon a tactic that fortuitously delivered the insects in the course of pursuing another objective. Such a coincidence of military tactics and entomological luck would seem incredible, but this unlikely convergence triggered an event more than six centuries ago that serves to warn the modern world of the horrific potential of biological warfare.
The conceptual roots of the apocalyptic conflict can be traced to the castle at Thun l'Eveque in Northern France.7 This was a gorgeous setting in which to conduct a gory experiment in military tactics. An English king, Edward III, laid claim to the French crown, which seems to be a great way of starting a fight—in this case, the Hundred Years War. To follow up his claim, Edward led an expeditionary force to the continent and took Thun l'Eveque castle. He left behind enough soldiers to defend the stronghold, but the troops soon grew restless. For want of entertainment, they took to harassing the residents in the nearby city of Cambrai. The residents were none too happy with the unprovoked raids and sought assistance from the Duke of Normandy. The French duke's forces laid siege to the castle and used "dyverse great engyns" (catapults and launching devices) for heaving rocks and whatnot into the English garrison. In this case, the nature of the "whatnot" is the key to the historical importance of the battle.
Firsthand accounts during the onslaught revealed that, along with boulders, they "cast in deed horses, and beests stynking, wherby they within had great[er] dystress thane with any other thynge."8 The motive behind heaving rotting carcasses has been roundly debated. Some scholars suggest that the offal was used to fill in during shortages of geologic ammunition. This explanation seems a bit implausible—were dead horses really more plentiful than rocks? Others propose that the Normans were just disposing of bodies, which also seems peculiar. After all, a dead horse makes a much better meal than it does a projectile—at least before it becomes a "stynking beest." The most compelling theory is that the carcasses were intended to induce illness, and not just the nausea evoked as rotten flesh plopped into the streets. Disease was thought to be promulgated from the stench associated with decomposition (recall that "bad air" was blamed for malaria). In any case, the defenders abandoned the castle after a relief force from England failed to dislodge the duke's troops.
The launching of dead livestock was not a common wartime practice, in substantial part because there were few catapults capable of heaving a thousand pounds of meat, and cutting a decomposing horse into smaller chunks was presumably beyond the call of duty. However, there were some huge tre-buchets, the successors of the catapults. These later devices worked by dropping a counterweight—as much as 50 tons—that was attached to the short end of a lever-arm, with the payload placed in a sling at the other end. Such "great engyns" were uncommon, but a more modest trebuchet could launch a sizeable carcass. While decomposing animals were nasty enough, a far more potent package was sometimes available.
The most devastating act of biological warfare in human history relied on sending insect-infested payloads into a besieged city. As with other uses of vectors during this period of history, the assailants were unaware of the instrumental role that the insects played in the success of their military innovation. Nor could the commander who ordered the assault ever have imagined that his entomological weapon of mass destruction would take 200 times more lives than the atomic bomb dropped on Hiroshima six centuries later.
Kaffa was a Genoese seaport, strategically positioned on the Crimean peninsula of the Black Sea.9 The city was a thriving hub, linking the maritime commerce of the Mediterranean, the overland caravans of the Far East, and the river trade extending up the Don and Volga into Moscow. After the Mongols razed the city in 1308, it was rebuilt and fortified with two concentric walls. Thirty-five years later, the Tartar army again descended on the region and captured Tana, a trading center east of Kaffa. The Venetian merchants fled to the neighboring stronghold with the Mongols hot on their heels. The army was led by Janibeg, the last Khan of the Golden Horde, that portion of the Mongol empire established by Ghengis Khan's grandson, Batu Khan.
The ensuing siege devolved into a brutal stalemate. Janibeg's forces were relentless, but Kaffa's walls proved impregnable—and the Genoese maritime hegemony ensured that the city could be provisioned. After the conflict had been deadlocked for three long years, a conspiracy of fleas, rats, and bacteria tipped the balance in favor of Kaffa. Or so it appeared at first. The Mongol camp was devastated by an outbreak of bubonic plague. The disease most likely came along with flea-infested rats—stowaways in supplies from the Eurasian steppe.
When the khan saw his army melting away in agony, he had no choice but to break the siege. Janibeg sealed his infamous place in history with one final order. Less than a decade after the Normans had catapulted dead horses, the Khan of the Golden Horde upped the ante of biological warfare. An eyewitness, Gabrielem de Mussis, reported the maneuver that changed the course of western history:
The Tartars, fatigued by such a plague and pestiferous disease, stupefied and amazed, observing themselves dying without hope of health, ordered [human] cadavers placed on their hurling machines and thrown into the city of Caffa, so that by means of these intolerable passengers the defenders died widely. Thus there were projected mountains of dead, nor could the Christians hide or flee, or be freed from such disaster.10
The spread of plague from the Mongol camp is unquestioned, but exactly how the disease made its way into the Genoese city is a matter of lively debate.11 Some scholars contend that flying corpses were not the vectors; instead, they propose that fleas were exchanged among rats in the no-man's-land between the opposing sides. However, fleas do not typically hopscotch between rats. Moreover, the gap between populations would have worked against such mixing. Given 14th-century siegecraft, the distance from the Mongol camp to the city walls was probably at least half a mile, with the front line as close as 200 to 300 yards—within the range of a trebuchet but beyond the range of Kaffa's archers. The typical home range for a rat is only 50 yards, far short of the distance between the human settlements. So exactly what component of the hideous payloads triggered the epidemic?
The infected corpses were a potential source of direct transmission, as the pathogen can be acquired by contact with infected tissue. Bacteria-laden buboes would presumably rupture on impact, providing a rich source of contagion. In the course of clearing these gruesome remains from the streets and rooftops, the residents might have become infected via cuts on their hands. A few people could have acquired the disease in this manner, but epidemiology is a numbers game. Whether the frequency of direct infection would have been sufficient to spark a full-fledged epidemic is doubtful. Plague has a far more efficient means of infection.
The bacteria, Yersinia pestis, that cause bubonic plague are transmitted by fleas in an elegantly evolved process.12 The pathogen is acquired when the insect feeds on an infected mammal. Some 30 species of fleas and more than 200 species of rodents can harbor the bacteria. Rats are most relevant to human outbreaks, with the rat flea, Xenopsylla cheopsis, and the human flea, Pulex irritans, being the most common one-two punch. The bacilli multiply in the stomach of the flea and create the perfect conditions for their own transmission. The microbes form a gelatinous mass in the upper digestive system of the flea. So when the blood-thirsty insect bites a host, rather than being able to suck a liquid meal into its stomach, the insect regurgitates bacteria into the bloodstream of the host. This blockage means that the hungry flea is condemned to repeatedly seek out and bite hosts in futile attempts to feed—while transmitting the pathogen to another victim in the course of each frustrated meal.
Within three or four days of being bitten, a human victim is wracked with fever and excruciating headaches. Then, the lymph glands swell to painful proportions, forming the infamous buboes in the armpits and groin. Conditions deteriorate as the bacteria spread to the blood stream, liver, and spleen. In short order, delirium mercifully gives way to a coma, with seizures and bleeding from various orifices portending the end. In some cases, a secondary infection occurs in the lungs. From here, a pulmonary form of the disease develops, and "pneumonic plague" can then be transmitted by infective droplets. However, we can be quite sure that the bodies being hurled into Kaffa were in no condition for sneezing or coughing.
The only trouble with holding the fleas solely responsible for the epidemic in Kaffa is that these insects don't hang around long after their host's death. However, we can posit that a portion of the catapulted bodies were reasonably fresh, and the insects don't bail out immediately—even a determined flea can't move very quickly within the folds of a person's clothing. Given the efficiency of pathogen transmission, not many fleas would have been needed to spread disease among the city's rats, dogs, cats—and humans. The Mongol khan clearly meant to spread the suffering of his troops into the streets of Kaffa, but what came next was surely unimaginable even to the most vengeful martial mind.
The Genoese evacuated in a desperate attempt to escape the epidemic. Healthy and sick people, along with the usual complement of stowaway rats and their attendant fleas, headed out to sea. Within a year, the ships from Kaffa had sailed across the Black Sea to Constantinople, then along the Mediterranean to the islands of Sicily, Sardinia, and Corsica, and finally to the cities of Genoa, Venice, and Marseilles. At each port, infected people, rats, and fleas disembarked, seeding southern Europe with plague. Disease spread through Italy and France into the heart of the continent, and by 1350 all of Europe was embroiled in the pandemic. The Black Death took two-thirds of the population of Hamburg, half of the population of Florence, and 1.7 million of England's 3.8 million inhabitants. Twenty-five million people, more than a quarter of Europe, became inadvertent victims—collateral casualties, in modern parlance—of Janibeg's parting shot on Kaffa.13
We can vilify the Mongol khan, but this is not entirely fair, given that his tactic was repeated in subsequent centuries, albeit without the large-scale consequences. In 1422, Prince Sigismund's forces launched corpses (along with manure and garbage) during the siege of Karlstein, but no epidemic ensued and it is not clear whether the bodies were diseased.14 The siege of Reval, however, had all but one of the ingredients needed to become a repeat of Kaffa.
At the turn of the 17th century, Karl XII of Sweden was on a kingly tear across northern Europe to acquire as much territory as possible.15 His acquisitive rampage ground to a halt when the Russians counterattacked and forced a portion of Karl's forces to hole up in the Estonian city of Reval. The Russians were beset by the plague and soon resorted to hurling corpses. As in Kaffa, the tactic worked—plague irrupted within the walls of Reval. However, the dying populace had no way to escape, and the epidemic was contained within the city limits.
By this time, human warfare was changing and so was the role of insects. Although cannons and gunpowder shifted attention to exploding—rather than living (or dead)—payloads, insects flourished with the new military strategies. Vector-borne disease played a decisive role in the course of many human conflicts in the 18th and 19th centuries, although the combatants were largely unaware that six-legged soldiers were often far more deadly than their human opponents. The proboscis proved mightier than the sword—or the cannon. It was as if, having been conscripted and exploited for centuries, the insects declared their independence and fought for themselves. And they didn't lose a battle for two hundred years.
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