Death toll of smallpox and cocoliztli pandemics in Mexico 1519-1578

Following the arrival of Spanish colonizers in 1519, namely Hernando Cortes and his 600 conquistadors, the indigenous population of the Mexican valley saw a dramatic decline. In the first two years of conquest, thousands of indigenous Americans perished while fighting the European invaders, including an estimated 100,000 who died of violence or starvation during Cortes' siege of the Aztec capital city, Tenochtitlan (present-day Mexico City), in 1520. However, the impact of European violence on population decline pales in comparison to the impact of Old World diseases, which saw the indigenous population of the region drop from roughly 22 million to less than two million within eight decades..

Virgin soil pandemics

Almost immediately after the Spanish arrival, a wave of smallpox swept across the indigenous populations, with some estimates suggesting that five to eight million natives died in the subsequent pandemic between 1519 and 1520. This outbreak was not an isolated incident, with the entire indigenous population of the Americas dropping by roughly ninety percent in the next two centuries. The Mexican valley specifically, which was the most populous region of the pre-Columbian Americas, suffered greatly due to virgin soil pandemics (where new diseases are introduced to biologically defenseless populations). In the Middle Ages, the majority of Europeans contracted smallpox as children, which generally granted lifelong immunity. In contrast, indigenous Americans had never been exposed to these diseases, and their populations (of all ages) declined rapidly.

Cocoliztli

Roughly three decades after the smallpox pandemic, another pandemic swept across the valley, to a more devastating effect. This was an outbreak of cocoliztli, which almost wiped out the entire population, and was followed by a second pandemic three decades later. Until recently, historians were still unsure of the exact causes of cocoliztli, with most hypothesizing that it was a rodent-borne disease similar to plague or an extreme form of a haemorrhagic fever. In 2018, however, scientists in Jena, Germany, studied 29 sets of teeth from 16^th century skeletons found in the Oaxaca region of Mexico (from a cemetery with known links to the 1545 pandemic); these tests concluded that cocoliztli was most likely an extreme and rare form of the salmonella bacterium, which caused paratyphoid fever. These pandemics coincided with some of the most extreme droughts ever recorded in North America, which exacerbates the spread and symptoms of this disease, and the symptoms described in historical texts give further credence to the claim that cocoliztli was caused by salmonella.