Answer based on context:

A single soldier arriving in Mexico in 1520 was carrying smallpox and thus initiated the devastating plagues that swept through the native populations of the Americas. The European diseases that ravaged the indigenous inhabitants of the Americas also severely affected the various Maya groups of Petén. It is estimated that there were approximately 30,000 Ch'ol and Ch'olti' Maya in western Petén at the start of the 16th century. Between 1559 and 1721 they were devastated by a combination of disease, war and enforced relocations. When Nojpetén fell in 1696 there were approximately 60,000 Maya living around Lake Petén Itzá, including a considerable number of refugees from other areas. It is estimated that 88% of the inhabitants died during the first decade of colonial rule because of disease and war. Although disease was responsible for the majority of deaths, internecine warfare between rival Maya groups and Spanish expeditions also took their toll.

What killed most Mayans in the first decade of colonial rule?
disease and war