These are parallel to shifts that occurred in Europe such as the creation of large urban areas. Since city planning wasn't what it is today, cities were overcrowded, sewers were nonexistent or inefficient, and disease carrying vermin multiplied. This created a welcome mat for infectious disease in addition to the general uncleanliness of the population and the great number of transient people such as soldiers, students, thieves and the mentally ill. Another factor leading to the assault of disease on medieval Europe was the domestication of large mammals. These animals were the origins of some of the most cursed afflictions of the time.
Smallpox is a derivative of cowpox, measles of canine distemper, and influenza of hog diseases.
At first, neither young or old were spared. After generations, susceptible individuals were eliminated and resistant survivors dominated the gene pool. Diseases went from epidemics to childhood ills. Smallpox made its American debut in 1519, when it struck the Caribbean island of Santo Domingo, killing up to half of the indigenous population. From there outbreaks spread across the Antilles islands, onto the Mexican mainland, through the Isthmus of Panama and into South America.