Why did disease help the Spanish defeat the Aztec natives?

Epidemics are not a recent phenomenon. Infectious diseases have wreaked havoc on human populations throughout history. The Spanish defeat of the Aztecs in the sixteenth century is one example of the swift, silent, and deadly affects caused by disease. It took only two years for Hernán Cortés to conquer and seize the Aztec empire for Spain. Cortés sailed from Cuba to Mexico in February 1519 with the intention of exploring and colonizing the region. By 1521, the Aztec ruler Montezuma had died, the capital city of Tenochtitlan was captured, and the empire formerly ruled by the Aztec peoples was under Spanish reign. The Spanish weapons, war tactics, and Cortés’s leadership abilities were undoubtedly a factor, but it was smallpox that was the most devastating to the Aztec population.
Native Americans such as the Aztecs had never come in contact with Old World diseases such as smallpox and therefore possessed no natural immunity to the virus; furthermore, no effective anti-viral therapies were available to treat it. Smallpox killed many of its Aztec victims from the onset of exposure, especially those who were more vulnerable, such as infants and young children. Many others were weakened by the virus, making them ill and making it more difficult to care for themselves and others in the community while fending off the Spanish invaders. In addition, with so many people in a debilitated state, they could not tend to crops or find other sources of food, which caused widespread famine and further diminished the immune systems of those who had survived the epidemic and were recovering.
https://www.pastmedicalhistory.co.uk/smallpox-and-the-conquest-of-mexico/

https://www.pbs.org/newshour/science/how-smallpox-devastated-the-aztecs-and-helped-spain-conquer-an-american-civilization-500-years-ago