indicate that the population was so devastated by disease that they could hardly resist the foreign forces. Historians differ as to whether the illness of the 1520s was smallpox; a minority of scholars claim that the epidemic was due to an indigenous illness called Carrion's disease. In any case, a 1981 study by N. D. Cook the shows that the Andes suffered from three separate population declines during colonization. The first was of 30–50 percent during the first outbreak of smallpox. When a measles outbreak occurred, there was another decline of 25–30 percent. Finally, when smallpox and measles epidemics occurred together, which occurred from 1585 to 1591, a decline of 30–60 percent occurred. Collectively these declines amounted to a decline of 93 percent from the pre-contact population in the Andes region. Mortality was particularly high among children, ensuring that the impact of the epidemics would extend to the next generation. Beyond the devastation of the local populations by disease, they suffered considerable enslavement, pillaging and destruction from warfare. The Spanish took thousands of women from the local natives to use as servants and concubines. As Pizarro and his men took over portions of South America, they plundered and enslaved countless people. Some local populations entered into vassalage willingly, to defeat the Inca. Native groups such as the Huanca, Cañari, Chanka, and Chachapoya fought alongside the Spanish as they opposed Inca rule. The basic policy of the Spanish towards local populations was that voluntary vassalage would yield safety and coexistence, while continued resistance would result in more deaths and destruction. Another significant effect on the people in South America was the spread of Christianity. As Pizarro and the Spanish subdued the continent and brought it under their control, they forcefully converted many to Christianity, claiming to have educated them in the ways of the "one true religion." With the depopulation of the local populations along with the capitulation of the Inca Empire, the Spanish missionary work after colonization began was able to continue unimpeded. It took just a generation for the entire continent to be under Christian influence. Environmental impact The arrival of the Spanish also had an unexpected impact on the land itself, recent research points out that Spanish conquest of the Inca altered Peru's shoreline. Before the Spaniards arrived, inhabitants of the arid northern Peruvian coast clad massive sand dune–like ridges with a -likely- accidental form of “armor”, millions