Smallpox in the New World: Vignettes featuring Hernan Cortes, Francisco Pizarro, and Lord Jeffrey Amherst

Smallpox was one of the greatest scourges in human history. Before it was pronounced to be officially eradicated in 1977, smallpox was estimated to have killed, crippled, or maimed nearly 1/10 of all individuals who ever lived. During the 18th century in Europe, smallpox killed, on average, about 400,000 persons per year. In fact, even during the 20th century, before worldwide vaccination led its eradication, smallpox is believed to have killed more than 300 million people! [The last documented smallpox case worldwide occurred in Somalia in 1977. The last case in the United States was reported in 1949.]

 Notwithstanding the decimation smallpox wrought in European populations, which had several thousand years to adapt to it, smallpox was even more devastating to Native Americans, who were exposed to it only after European explorers, conquerors, and colonizers brought it to the New World. The Europeans also brought other diseases to the New World; most significantly, measles, influenza, typhus, and bubonic plague. Yet smallpox was the most devastating of the European infectious diseases to the indigenous people of the New World. Importantly, smallpox readily spread throughout the Americas, decimating Native American populations, before most had ever actually made contact with the Europeans themselves.

Estimates of how many Native Americans were living in the Americas when Columbus arrived, and how many may have succumbed to smallpox and other Old World diseases, vary considerably. Some estimates claim that 95 percent of the pre-Columbian Native American population succumbed to these Old World diseases. At any rate, the ruin caused by these diseases among the Native American populations was unquestionably enormous. Moreover, this devastation continued into the 20th Century, particularly among the Alaskan Inuit peoples, as well as the native populations of Australia, New Guinea, and Africa.

Bearing the above in mind, we now consider that Hernan Cortes and Francisco Pizarro each came to the New World, in the early 16th century, with an entourage numbering a mere several hundred or less. Yet each conquered a fierce warrior empire numbering in the millions. Cortes conquered the Aztecs and Pizarro, the Incas. How were the Spanish conquistadores able to vanquish these empires, in the face of such overwhelming numerical odds?

Some historians attribute the Spanish conquests of the Aztecs and Incas to their steel weapons and armor and, even more so, to their horses. Others attribute the Spanish victories to the devastating effect of smallpox on those Native American civilizations. Thus, a key purpose of this posting is to sort through these differing points of view, to give each standpoint its proper due. Then, noting that smallpox was an inadvertent factor in the conquests of the Aztecs and Inca empires by the Spanish conquistadores, we recount how British forces in colonial North America, led by Lord Jeffrey Amherst, deliberately used smallpox as a bio-weapon against rebelling Native Americans, in the first documented instance of biological warfare in the New World. [To place these events in time, recall that Columbus first landed in the New World in 1492. Cortes’ encounter with the Aztecs happened a mere 27 years later, in 1519. Pizarro’s encounter with the Incas happened only 13 years after that, in 1532. Amherst’s episode happened in what is now western Pennsylvania, more than 200 years later, in 1763.]

John Keegan, a well known writer on military history, believes that the Spaniards’ horses were the foremost factor in their conquests of the Aztecs and Incas. These Native Americans had never seen horses before they confronted the horses ridden into battle by the conquistadores. Thus, we might very well appreciate how shocking it must have been to Aztec and Inca foot soldiers, when facing a charging horse for the first time. Moreover, horses gave the Spaniards tremendous advantages of speed and maneuverability on the battlefield. Hence, Keegan states, “…in a contest of hundreds against thousands, it was their horses that gave the invaders the decisive (my emphasis) advantage.”

Yet Cortez started out with only 17 horses and Pizarro with only 27. Thus, could so few horses have actually been the factor that enabled several hundred Spanish conquistadors to defeat warrior empires of several millions? In support of this premise, there were numerous battles, in which a mere few dozen or less Spanish horsemen routed thousands of Aztec and Inca warriors, while slaughtering many of them in the process.

Keegan also credits another factor with regard to the defeat of the Aztecs; the extraordinary limitations the Aztecs imposed on their own war-making ability; at least by European standards. Although Aztec armies were very well trained, organized, and supplied, the objective of Aztec warfare was the taking of large numbers (many thousands in some instances) of live prisoners for their ritual sacrifices. Consequently, Aztec weapons and tactics were designed to wound and immobilize, rather than to kill. Thus, while the Aztecs had bows and arrows, their favored weapon was a wooden sword, studded along its sides with flakes of flint, which was designed just to wound. The warrior’s objective was to close with an opponent and strike a disabling blow to his legs, thereby crippling him and enabling his capture. And, since Aztec battles were fought for the purpose of taking prisoners, they were characterized by a high degree of ceremony and rituals. Moreover, the fighting effectiveness of their enemies was similarly limited by the same ceremony and rituals. Battles were prearranged, and the fate of the captives was known in advance. Remarkably, it was all part of a culture in which prisoners were expected to be voluntary participants in their own ritual murders. What’s more, the Aztecs could engage in such ritualized warfare because they were not challenged at their borders by any existential threat. Thus, Aztec weapons, strategy, and tactics were hardly suited for battle against the invading Europeans, whose sole purpose was to win a decisive crushing victory.

Nonetheless, while the Spanish had the advantages of horses, and superior weapons and tactics, could these factors alone have prevented the Aztecs from simply overwhelming them by weight of their sheer numbers alone. Indeed, the Aztecs nearly did just that in their first encounter with the conquistadores, in the Aztec capital, Tenochtitlan; at the time bigger and richer than any city in Spain. The conflict began when Cortes, famously and suddenly took the Aztec emperor, Montezuma, prisoner in his own palace. Despite the fact that the Aztecs were stunned by Cortes’ audacity, his assault on the now leaderless Aztecs nearly ended in disaster for him. Cortes lost two-thirds of his force, and was barely able to escape from Tenochtitlan, and then retreat back to the coast.

Cortes’ stroke of good fortune came after his failed attempt to capture Tenochtitlan. It was the chance introduction of variola (the smallpox virus), which spread rapidly through the densely populated Aztec empire, killing a third or more of its population in a mere few months. The smallpox victims included Cuitlahuac, the Aztec emperor who succeeded Montezuma. After that, in 1521, Cortes attempted to subdue Tenochtitlan a second time. In this instance, Cortes’ was reinforced with a large number of Indian auxiliaries (perhaps as many as 200,000). But, the defending Aztecs were no longer naïve regarding Spanish weapons and intentions, and they fought back tenaciously. Nevertheless, smallpox had been taking its inevitable toll, by then having killed nearly half of the Aztecs, and Cortes was able to capture the city.

Exactly how smallpox came to Mexico is not entirely clear. Some sources state that an infected slave, who arrived in Mexico in 1520 from Spanish Cuba, transmitted the infection to the Aztecs. Other accounts suggest that smallpox was carried by Cuban Indians, who the Spaniards brought along as auxiliaries. Regardless, since the Aztecs had no prior exposure to variola, most of them and their leaders were killed by the Old World germ, leaving the survivors bewildered and demoralized. It is estimated that 3.5 million Aztecs succumbed to smallpox in a mere two years, vastly exceeding the number that possibly could have been killed by Spanish guns and swords!

Shortly after Cortes’ conquest of the Aztecs in Mexico, Francisco Pizarro, in 1532, with a mere 168 Spanish soldiers, and only 12 guns, which were the slow-to- load, inaccurate harquebuses of the day, conquered the Inca Empire of millions. The first encounter between Pizarro and the Incas was at Cajamarca, a town in what is now the Peruvian highlands. At Cajamarca, Pizarro’s force of 168 men faced, and soundly routed an Inca army of 80,000, without losing a single man! As in the case of Cortes at Tenochtitlan, Pizarro, at Cajamarca, enjoyed the advantage of his horses (27 in this instance) and superior weaponry. What’s more, following the example set by Cortes’ capture of Montezuma, Pizarro captured the Inca emperor, Atahualpa, moments after he unleashed his surprise attack on the shocked Inca warriors.

To wholly grasp the audacity of Pizarro’s achievement at Cajamarca, reconsider that Atahualpa was surrounded by his army of 80,000 soldiers, in the middle of his own empire of millions, while Pizarro’s force was comprised of a mere 168 men. What’s more, Pizarro was isolated from any other Spaniards, the nearest of whom were 1,000 miles to the north, in Panama.

Jared Diamond, in his marvelous book, Guns, Germs, and Steel, provides gripping first-hand Spanish accounts of the confrontation at Cajamarca. One of the conquistadores relates that many of the Spanish soldiers, when first seeing the enormous numerical advantage of the Incas, were terrified to the point of incontinence. He continues: “All of us were full of fear, because we were so few in number and we had penetrated into a land where we could not hope to receive reinforcements…The Governor’s brother Hernando Pizarro estimated the number of Indian soldiers there at 40,000, but he was telling us a lie just to encourage us, for there were more than 80,000 Indians…At noon Atahualpa began to draw up his men and approach. Soon we saw the entire plain full of Indians, halting periodically to wait for more Indians who kept filing out of the camp behind them. They kept filing out in separate detachments into the afternoon. The front detachments were now close to our camp, and still more troops kept issuing from the camp of Indians. In front of Atahualpa went 2,000 Indians who swept the road ahead of him, and these were followed by the warriors, half of whom were marching in the fields on one side of him and half on the other side.”

Another eyewitness describes Pizarro’s actual attack on the Inca force: “We had placed rattles on the horses to terrify the Indians. The booming of the guns, the blowing of the trumpets, and the rattles on the horses threw the Indians into panicked confusion…The Governor (Pizarro) himself took his sword and dagger, entered the thick of the Indians with the Spaniards who were with him, and with great bravery reached Atahualpa’s litter. He fearlessly grabbed Atahualpa’s left arm…The Indians carrying the litter, and those escorting Atahualpa never abandoned him: all died around him…Atahualpa himself admitted that we had killed 7,000 of his men in that battle…It was extraordinary to see so powerful a ruler captured in so short a time, when he had come with such a mighty army.”

Our eyewitness continues: “The panic-stricken Indians remaining in the square, terrified at the firing of the guns and at the horses-something they had never seen-tried to flee from the square by knocking down a stretch of wall and running out onto the plain outside. Our cavalry jumped the broken wall and charged into the plain… It was an astonishing sight, for the whole valley for 15 or 20 miles was completely filled with Indians. Night had fallen, and our cavalry were continuing to spear Indians in the field, when we heard a trumpet calling for us to reassemble at camp.”

From the above eyewitness accounts, and others as well, we know that horses indeed played an important role in the Spanish conquests of the Aztecs and Incas. Apropos that, David Diamond notes that the only instances where Native Americans were able to stave off being subdued by European invaders and settlers for any length of time were when they were able to acquire horses of their own and guns as well, and master their use (e.g., the Sioux warriors who annihilated Custer’s forces at the Little Big Horn in 1876).  The Aztecs and Incas, like all other foot soldiers, were never able to defeat cavalry in the open.

Actually, we’ve said only little regarding guns in the conquest of the Aztecs and Incas. That is so because the guns of that day were difficult to load and fire, and were inaccurate as well, as noted above. Thus, aside from their ability to induce panic, guns played only a minor role in those conflicts. Instead, Spanish steel swords, lances, body armor, and helmets, were more important than their guns, when pitted against the Indian’s blunt wooden clubs and thin quilted armor. [The Sioux warriors at the Little Big Horn had accurate, easy to load, repeating rifles that, in point of fact, were superior to the single-shot rifles carried by Custer’s cavalry.]

We’ve said nothing thus far regarding smallpox in the downfall of the Incas. To redress that point, note that while some of the details of the encounter between Atahualpa and Pizarro at Cajamarca, as recounted above, may be familiar to many readers, less well known is the reason Atahualpa was at Cajamarca, rather than at Cuzco, the capital of the Inca Empire. The reason is a follows. In 1526, a smallpox epidemic, which was brought to the New World by Spanish settlers in Panama and Columbia, spread overland to the Inca Empire, killing the then emperor Huayna Capac and his son and heir, Ninan Cuyuchi. Many other Incas were killed as well. But, for the moment, the gap in Inca leadership, caused by the deaths of Huayna Capac and Ninan Cuyuchi, gave rise to a contest for power between Huayna Capac’s two other sons, Atahualpa and Huascar. Atahualpa, and his army, was at Cajamarca because he had just won a decisive battle against Huascar’s forces.

From the time of the battle at Cajamarca, when Pizarro captured Atahualpa, at until eight months later when the Spaniards executed him, the Incas would not take any offensive action against the Spaniards; for fear that doing so might place their revered sun-god emperor in jeopardy. Pizarro used this respite to arrange for reinforcements from Panama. Then, when hostilities resumed after Atahualpa’s execution, the Spaniards were in a much stronger position militarily against the Incas, who were by then decimated by smallpox, as well as by their civil war. What’s more, because Inca society was very much connected to its emperor, Atahualpa’s death further hastened its disintegration and, thus, the ultimate defeat of the empire by the conquistadores.

Before moving on, note that smallpox was a non- premeditated, chance factor in the conquests of the Aztec and Inca Empires by the Spanish conquistadores. In contrast, smallpox was used deliberately against Native Americans by the British military in colonial North America, in what was the first known example of biological warfare in the New World. But, before we begin that story per se, note that biological warfare was not a new concept. Indeed, there is evidence that it was practiced as early as the 6th Century B.C., when the Assyrians were said to have poisoned enemy wells with a fungus that was supposed to make the enemy delusional. Irrespective of whether this particular tactic might actually have worked, the concept was nevertheless in existence. Later, in medieval Europe, bubonic plague victims and their excrement were catapulted over castle walls. The last known use of plague corpses being used as a bio-weapon occurred in 1710, when Russian forces flung plague-infected corpses over the city walls of Reval, the capital city of Estonia. Incidentally, despite the devastation wreaked by natural smallpox infections over the course of several millennia, the earliest example that I found of smallpox actually being used as a bio-weapon is that which follows.

Our current tale features Lord Jeffrey Amherst, for whom my home-town in Massachusetts is named. Amherst was the commanding general of British forces in North America during the final battles of the so-called French and Indian war (1754-1763). The French and Indian War was the American theater of a much larger conflict playing out in Europe, known as the Seven Years War. In the New World, the British and French vied for domination over North America. The British were victorious, but at great cost to them. Their heavy taxes on the colonies, to recover the costs of the war, ultimately led to the American war of independence only 13 years later.

Amherst led military victories over the French forces that were critical to the British winning control over all of North America. Nevertheless, Amherst’s reputation is tarnished by the belief that he deliberately gave smallpox-infected blankets to North American Indians, thereby starting a deadly epidemic among them.

These events happened just after the French and Indian war, when relations between the British and Native Americans in the Ohio and Great Lakes region began to deteriorate, leading to the 1763 Indian uprising known as Pontiac’s Rebellion. The conflict is named for the Ottawa chief, Pontiac, who led a coalition of tribes in an attempt to drive British forces from the region. The Ohio and Great Lakes region was previously occupied by the French, who had been courting the Native Americans’ favor. However, when British forces took control, they treated the Native Americans as a conquered people.

Amherst believed that the Native Americans should have to unconditionally accept British rule. But, as Pontiac’s forces seized the military initiative in the Ohio and Great Lakes region, Pennsylvanian colonists sought refuge at Fort Pitt (located at what is now Pittsburgh). Next, after warriors of the Delaware tribe laid siege to the fort, Amherst wrote the following to the British colonel who was about to lead an expedition to relieve it: “Could it not be contrived to send the small pox among the disaffected tribes of Indians? We must on this occasion use every stratagem in our power to reduce them.” The colonel wrote back to Amherst in agreement, suggesting blankets as the vector by which to transmit the contagion. Amherst replied: “You will do well to inoculate the Indians by means of blankets, as well as every other method that can serve to extirpate this execrable race.”

While Amherst is generally regarded as the villain of this episode, there is evidence that the British commander at Fort Pitt (the Swiss-born captain, Simeon Ecuyer) had already, and independently of Amherst, attempted this very tactic, by giving representatives of the besieging Indians blankets and a handkerchief that were deliberately exposed to smallpox at the fort’s hospital. This is not meant to exonerate Amherst, whose intentions and hatred of Native Americans are clear from his own correspondences. Nevertheless, in fairness to Amherst, we need to view his attitudes and actions in the context of his own times.  While eighteenth century European rules of warfare indeed had strictures against the use of “poison” weapons and the “poisoning” of streams, springs, and wells, they nevertheless allowed for excessive brutality when putting down rebellions or against populations or groups regarded as heathens or savages. That was the code of warfare of the day, to which Amherst adhered. Apropos that, Amherst never showed any obsessive desire to “extirpate” his other enemy, the French. Yet irrespective of who may have initiated the attempt to spread smallpox among the Native Americans besieging Fort Pitt, this was the first documented example of deliberate germ warfare in North America.

We might ask whether this tactic on the part of the British at Fort Pitt actually worked. But, historians writing on this issue have not come to a consensus for several reasons; most importantly because smallpox was already present among the Native Americans in the region, before the start of their rebellion. At any rate, in the spring and summer of 1763, the Indians around Fort Pitt were stricken with smallpox.

Some final points on bio-weapons:

First, whereas bio-weapons have been used throughout the past three millennia, many contemporary military experts believe that infectious agents would be of little use on a modern battlefield. One reason is that unlike nuclear, chemical, and conventional weapons, they would not immediately stop an advancing army. Another reason is that once released, the spread of a bio-agent would be virtually impossible to control. Finally, use of bio-weapons would invite retaliation in kind. Thus, in the contemporary world, bio-weapons are feared mainly for their potential use by terrorist groups. In this regard, when used as a terror agent, an infectious bio-weapon does not need to cause an epidemic to cause widespread panic and disruption. This was shown by the 2001 episode in the United States, in which B. Anthracis was sent through the postal system.

Second, of the many pathogenic microorganisms that might be used by terrorists, most biological warfare experts believe that smallpox and Bacillus anthracis pose the major threats. Bacillus anthracis offers the advantage of being highly stable in the environment. The advantages of smallpox are that it can spread very rapidly from person-to-person, it is difficult to diagnose until the variola infection is well underway in an individual, it is highly virulent (killing about 30% of infected humans), and there is no effective treatment.

Third, until recently, any incentive to use smallpox as a bio-weapon was greatly diminished by the success of worldwide vaccination. However, this state of affairs began to change in 1980, three years after smallpox was declared to be eradicated. At that time,the World Health Assembly recommended an end to routine vaccination, and most countries complied. Thus, most individuals living today have never been vaccinated against smallpox, and it is not known for certain whether those who received vaccinations 25 or more years ago are still protected.

Fourth, and finally; because of the success of the 20th century’s smallpox eradication program, all reference stocks of variola virus in laboratories worldwide were destroyed, with the exceptions of those at the U.S. Centers for Disease Control and Prevention (CDC), and at Russia’ State Research Center of Virology and Biotechnology (VECTOR). What’s more, the former Soviet Union weaponized variola virus, in contradiction of the 1972 Biological and Toxin Weapons Convention. Because of concern that any of this remaining virus might somehow fall into the hands of terrorists, or inadvertently escape from these laboratories, and since rogue nations or terrorist groups may also be able to develop it as a bio-weapon, many have argued that these last remaining variola stocks should be destroyed. Yet a case can also be made for maintaining them (e.g., for continuing research on the virus). Perhaps this issue might be the topic of a future posting.



Guns, Germs, and Steel, by Jared Diamond, (W. W. Norton & Company, 1999)

1493, by Charles C. Mann, (Alfred A. Knopf, 2011)

 A History of Warfare, by John Keegan, (Alfred A. Knopf, 1993).

                The first two of these books were my principal sources for matters concerning the conquests of the Aztecs and Incas. Diamond’s book offers fascinating insights into human history in general and, apropos this posting, eyewitness accounts of the battle at Cajamarca. Mann’s book tells of human life in the Americas before the arrival of Columbus. It contains a detailed, spine-tingling description of Cortes’ capture of Tenochtitlan. Each of these books is very strongly recommended. Keegan’s book, which is a general history of warfare, gives an intriguing depiction of warfare in Aztec culture.

 Fenn, E.A., (2000) Biological Warfare in Eighteenth Century North America: Beyond Jeffrey Amherst, American Journal of History 86:1552-1581.

 This paper contains a detailed account of the events at Fort Pitt in 1763. It also notes other possible, but not as well documented examples of biological warfare in colonial North America.

Cotton Mather, Onesimus, George Washington, and Variolation, on the blog.

See this posting for more on smallpox in colonial North America.








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