Question: What disease was brought by the Spanish conquerors that ultimately killed millions of Aztecs 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.
s and notable epidemics through history.
There have been a number of significant pandemics recorded in human history, generally zoonoses which came about with domestication of animals, such as influenza and tuberculosis.
There have been a number of particularly significant epidemics that deserve mention above the "mere" destruction of cities:  Plague of Athens, 430 BC.
Typhoid fever killed a quarter of the Athenian troops, and a quarter of the population over four years.
This disease fatally weakened the dominance of Athens, but the sheer virulence of the disease prevented its wider spread; i.e.
it killed off its hosts at a rate faster than they could spread it.
The exact cause of the plague was unknown for many years.
In January 2006, researchers from the University of Athens analyzed teeth recovered from a mass grave underneath the city, and confirmed the presence of bacteria responsible for typhoid.
Antonine Plague, 165–180.
Possibly smallpox brought to the Italian peninsula by soldiers returning from the Near East; it killed a quarter of those infected, and up to five million in all.
At the height of a second outbreak, the Plague of Cyprian (251–266), which may have been the same disease, 5,000 people a day were said to be dying in Rome.
Plague of Justinian, from 541 to 750, was the first recorded outbreak of the bubonic plague.
It started in Egypt, and reached Constantinople the following spring, killing (according to the Byzantine chronicler Procopius) 10,000 a day at its height, and perhaps 40% of the city's inhabitants.
The plague went on to eliminate a quarter to a half of the human population that it struck throughout the known world.
It caused Europe's population to drop by around 50% between 550 and 700.
Black Death, started 14th century.
The total number of deaths worldwide is estimated at 75 million people.
Eight hundred years after the last outbreak, the plague returned to Europe.
Starting in Asia, the disease reached Mediterranean and western Europe in 1348 (possibly from Italian merchants fleeing fighting in Crimea), and killed an estimated 20 to 30 million Europeans in six years; a third of the total population, and up to a half in the worst-affected urban areas.
It was the first of a cycle of European plague epidemics that continued until the 18th century.
During this period, more than 100 plague epidemics swept across Europe.
In England, for example, epidemics would continue in two to five-year cycles from 1361 to 1480.
By the 1370s, England's population was reduced by 50%.
The Great Plague of London of 1665–66 was the last major outbreak of the plague in England.
The disease killed approximately 100,000 people, 20% of London's population.
Third Pandemic, started in China in the middle of the 19th century, spreading plague to all inhabited continents and killing 10 million people in India alone.
During this pandemic, the United States saw its first outbreak: the San Francisco plague of 1900–1904.
Today, isolated cases of plague are still found in the western United States.
Encounters between European explorers and populations in the rest of the world often introduced local epidemics of extraordinary virulence.
Disease killed part of the native population of the Canary Islands in the 16th century (Guanches).
Half the native population of Hispaniola in 1518 was killed by smallpox.
Smallpox also ravaged Mexico in the 1520s, killing 150,000 in Tenochtitlán alone, including the emperor, and Peru in the 1530s, aiding the European conquerors.
Measles killed a further two million Mexican natives in the 17th century.
In 1618–1619, smallpox wiped out 90% of the Massachusetts Bay Native Americans.
During the 1770s, smallpox killed at least 30% of the Pacific Northwest Native Americans.
Smallpox epidemics in 1780–1782 and 1837–1838 brought devastation and drastic depopulation among the Plains Indians.
Some believe that the death of up to 95% of the Native American population of the New World was caused by Old World diseases such as smallpox, measles, and influenza.
Over the centuries, the Europeans had developed high degrees of immunity to these diseases, while the indigenous peoples had no such immunity.
Smallpox devastated the native population of Australia, killing around 50% of Indigenous Australians in the early years of British colonisation.
It also killed many New Zealand Māori.
As late as 1848–49, as many as 40,000 out of 150,000 Hawaiians are estimated to have died of measles, whooping cough and influenza.
Introduced diseases, notably smallpox, nearly wiped out the native population of Easter Island.
In 1875, measles killed over 40,000 Fijians, approximately one-third of the population.
The disease devastated the Andamanese population.
Ainu population decreased drastically in the 19th century, due in large partto infectious diseases brought by Japanese settlers pouring into Hokkaido.
Researchers concluded that syphilis was carried from the New World to Europe after Columbus' voyages.
The findings suggested Europeans could have carried the nonvenereal tropical bacteria home, where the organisms may have mutated into a more deadly form in the different conditions of Europe.
The disease was more frequently fatal than it is today.
Syphilis was a major killer in Europe during the Renaissance.
Between 1602 and 1796, the Dutch East India Company sent almost a million Europeans to work in Asia.
Ultimately, only less than one-third made their way back to Europe.
The majority died of diseases.
Disease killed more British soldiers in India than war.
Between 1736 and 1834 only some 10% of East India Company's officers survived to take the final voyage home.
As early as 1803, the Spanish Crown organized a mission (the Balmis expedition) to transport the smallpox vaccine to the Spanish colonies, and establish mass vaccination programs there.
By 1832, the federal government of the United States established a smallpox vaccination program for Native Americans.
From the beginning of the 20th century onwards, the elimination or control of disease in tropical countries became a driving force for all colonial powers.
The sleeping sickness epidemic in Africa was arrested due to mobile teams systematically screening millions of people at risk.
In the 20th century, the world saw the biggest increase in its population in human history due to lessening of the mortality rate in many countries due to medical advances.
The world population has grown from 1.6 billion in 1900 to an estimated 7 billion today. 
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. 
During Cortés' absence, Pedro de Alvarado in Tenochtitlan obtained information that the Aztecs were about to attack him.
In response, de Alvarado ordered a preemptive slaughter of Aztec nobles and priests celebrating a festival in the city's main temple.
In retaliation, the Aztecs laid siege to the Spanish compound, in which Moctezuma was still being held captive.
By the time Cortés returned to Tenochtitlan in late June, the Aztecs had elected a new Hueyi Tlatoani named Cuitláhuac.
Cortés ordered Moctezuma to address his people from a terrace in order to persuade them to stop fighting and to allow the Spaniards to leave the city in peace.
The Aztecs, however, jeered at Moctezuma, and pelted him with stones and darts.
By Spanish accounts, he was killed in this assault by the Mexica people, though they claim he had been killed instead by the Spanish. 
It is often debated why the Aztecs took little action against the Spanish and their allies after they fled the city.
One reason was that Tenochtitlan was certainly in a state of disorder: the smallpox disease ravaged the population, killing still more important leaders and nobles, and a new king, Cuauhtémoc, son of King Ahuitzotl, was placed on the throne in February 1521.
The people were in the process of mourning the dead and rebuilding their damaged city.
Also, it is possible that the Aztecs truly believed that the Spanish were gone for good.
In any case, staying within Tenochtitlan as a defensive tactic may have seemed like a reliable strategy at the time.
This would allow them the largest possible army that would be close to its supplies, while affording them the mobility provided by the surrounding lake.
And any Spanish assault would have to come through the causeways, where the Aztecs could easily attack them.