Question: What was introduced by Spanish conquerors unintentionally that led to the death of millions of Aztecs starting in the 1520s?
About the article "Mexico", section "Conquest (1519)".

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Conquest (1519).
The Spanish conquest of the Aztec Empire began in February 1519 when Hernán Cortés arrived at the port in Veracruz with ca.
500 conquistadores, and later moved on to the Aztec capital.
On his search for gold and other riches, Cortés decided to invade and conquer the Aztec empire.
The ruler of the Aztec empire upon the arrival of the Spaniards was Moctezuma II, who was later killed; his successor and brother Cuitláhuac took control of the Aztec empire, but was among the first to fall from the smallpox epidemic a short time later.
Unintentionally introduced by Spanish conquerors, smallpox ravaged Mesoamerica in the 1520s, killing more than 3 million Aztecs.
Other sources, however, mentioned that the death toll of the Aztecs might have reached up to 15 million (out of a population of less than 30 million).
Severely weakened, the Aztec empire was easily defeated by Cortés and his forces on his second return.
Smallpox was a devastating and selective disease—it generally killed Aztecs but not Spaniards, who as Europeans had already been exposed to it for centuries and were therefore much more immune to it.
The deaths caused by smallpox are believed to have triggered a rapid growth of Christianity in Mexico and the Americas.
At first, the Aztecs believed the epidemic was a punishment from an angry god, but they later accepted their fate and no longer resisted the Spanish rule.
Many of the surviving Aztecs blamed the cause of smallpox to the superiority of the Christian god, which resulted in the acceptance of Catholicism and yielding to the Spanish rule throughout Mexico.
The territory became part of the Spanish Empire under the name of New Spain.
Mexico City was systematically rebuilt by Cortés following the Fall of Tenochtitlan in 1521.
Much of the identity, traditions and architecture of Mexico were created during the colonial period.
The aftermath.
Tenochtitlan, the Aztecs, and the Tlaxcalteca.
Tenochtitlan had been almost totally destroyed by fire and cannon shots.
Those Aztecs who survived were forbidden to live in the city and the surrounding isles, and they went to live in Tlatelolco.
Cortés imprisoned the royal families of the valley.
To prevent another revolt, he personally tortured and killed Cuauhtémoc, the last Aztec Emperor; Coanacoch, the King of Texcoco, and Tetlepanquetzal, King of Tlacopan.
The Spanish had no intentions of turning over Tenochtitlan to the Tlaxcalteca.
While Tlaxcalteca troops continued to help the Spaniards, and Tlaxcala received better treatment than other indigenous nations, the Spanish eventually disowned the treaty.
Forty years after the conquest, the Tlaxcalteca had to pay the same tribute as any other indigenous community.
Political.
Apparently, Cortes favored maintaining the political structure of the Aztecs, subject to relatively minor changes.
Religious.
Cortes immediately banned human sacrifice throughout the conquered empire.
Evangelization began in the mid-1520s and continued in the 1530s.
Many of the evangelists learned the native languages and recorded aspects of native culture, providing a principal source for our knowledge about them.
By 1560, more than 800 clergy were working to convert Indians in New Spain.
By 1580, the number grew to 1,500 and by 1650, to 3,000.
Economics.
The Spanish colonizers introduced the encomienda system of forced labor.
The indigenous peoples would be grouped onto collective-like plantations where they would become serfs.
The encomienda system successfully prevented any upward, social mobility for the indigenous peoples.
The Spanish also used forced labor, often outright slavery, in mining.
Indigenous peoples would be enslaved and often worked to death mining for gold and silver.
Spain discouraged foreign investment by countries other than Spain.
As a result, the Mexican economy remained relatively backwards and lethargic until the end of the Spanish colonization.  
After the conquest of Mexico, the Aztec emperor, Montezuma, offered Hernán Cortés and his companions fifty jars of foaming chocolate.
According to the account of Francisco Cervantes de Salazar, the great emperor had a stockpile of several thousand 'charges' (tens of thousands of cocoa "kernels").
The Italian Girolamo Benzoni in his book "La Historia del Mondo Nuovo" (1565) wrote that "...chocolate seemed more like a drink for pigs than a drink to be consumed by humans," noting that he had never tasted it despite residing there for over a year.
Despite these caveats, Gonzalo Fernández de Oviedo characterised it as an interesting ingredient, while showing some reluctance to describe how some Indians, after drinking it, had stained their lips as if they had ingested human blood.
The perceptions of the Spaniards were changing, in part, due to their increasing reliance on native ingredients.
The tortillas made with cornmeal or (tamales), heated without the use of fat did not appear to satisfy the tastes of the conquerors used to pork and culinary techniques based on frying in fat, or sauteed with liberal use of olive oil or bacon.
Foods popular in Spain at the time such as cheese were unknown to the inhabitants of the New World.
As the Spanish settlers began to run out the stocks they brought with them, they had to find substitute foods.
They therefore began to plant vegetables, such as chickpeas, cereals such as wheat and fruits like oranges or pears.
Additionally they introduced the cultivation of olives, grapes and sugar cane.
The latter ingredient became important.
From the end of the 16th century onwards, sugar cane began to be added to the cocoa paste, which led to greater acceptance of cocoa among the Spanish settlers.
During this settling in period, around the 1520s, the Spaniards had to get used to new foods and flavours while they attempted to adapt old world cultivation methods to the new climate.
Equally, however, the new ingredients brought by the Spanish settlers such as wheat and chickpeas struggled to find acceptance among the native populations who preferred their own homegrown dishes.
Those Spaniards from humble economic backgrounds often married richer Aztecs, often as concubines.
Thus, they tended to eat food influenced by Aztec gastronomy.
This hastened the spread of cocoa among both cultures.
Bernal Díaz del Castillo mentioned that in a banquet held at the Plaza Grande in Mexico (built on the ruins of the Aztec capital) to celebrate peace between Carlos I of Spain and Francis I of France chocolate was served in golden tablets.
The wide acceptance of cocoa by the Spanish conquistadors, especially the women, was also described by the Jesuit José de Acosta in his book "Historia natural y moral de las Indias" (published in 1590).
As a result, after the initial aversion to cocoa had disappeared, supplies were sent to Spain.
The second major transformation of chocolate at the hands of the Spanish was in the serving method: the cocoa was heated until it became a liquid.
This was in contrast to the natives of the New World, who generally drank it cold or at room temperature.
The third change was the addition of spices from the Old World like cinnamon, ground black pepper or aniseed. 
When it became more clear what was happening to the Aztecs outside the Temple, the alarm was sounded.
Aztec warriors came running, and fired darts and launched spears at the Spanish forces.
This may have been due to the fact that their military infrastructure was severely damaged after the attack on the festival, as the most elite seasoned warriors were killed.
Alvarado sent for word to Cortés of the events, and Cortés hurried back to Tenochtitlan on June 24 with 1,300 soldiers, 96 horses, 80 crossbowmen, and 80 arquebusiers.
Cortés also came with 2,000 Tlaxcalan warriors on the journey.
Cortés entered the palace unscathed, although the Aztecs had probably planned to ambush him.
The Aztecs had already stopped sending food and supplies to the Spaniards.
They became suspicious and watched for people trying to sneak supplies to them; many innocent people were slaughtered because they were suspected of helping them.
The roads were shut and the causeway bridges were raised.
The Aztecs halted any Spanish attacks or attempts to leave the palace.
Every Spanish soldier that was not killed was wounded.
Cortés failed to grasp the full extent of the situation, as the attack on the festival was the last straw for the Aztecs, who now were completely against Moctezuma and the Spanish.
Thus, the military gains of the attack also had a serious political cost for Cortés.
Cortés attempted to parley with the Aztecs, and after this failed he sent Montezuma to tell his people to stop fighting.
However, the Aztecs refused.
The Spanish asserted that Moctezuma was stoned to death by his own people as he attempted to speak with them.
The Aztecs later claimed that Montezuma was murdered by the Spanish.
Two other local rulers were found strangled as well.
Moctezuma’s younger brother Cuitláhuac, who had been ruler of Ixtlapalapan until then, was chosen to be the new Tlatoani. 
 Cuauhtémoc, the last Independent Emperor of the Aztecs, deposed by Spain 1521 died 1525.
Luis Nanacacipactzin was the last dynastic-lined ruler (under Spanish occupation) up to his death in 1565.