Martín de Ursúa used his conquest of the Itza as a stepping stone to achieving the coveted post of Governor-General of the Philippines, which he took up in 1709. European-introduced diseases devastated the native population of Petén, with the effects of disease compounded by the psychological impact of defeat. The population around Lake Petén Itzá numbered between 20,000 and 40,000 in 1697. By 1714, a census recorded just over 3,000 individuals in Spanish Petén, including non-Indians. This number would not have included the so-called "wild" Maya living in the forest far from Spanish administration and control. By 1700 the new colonial capital of Petén was mainly inhabited by colonists, soldiers and convicts. During the second half of the 18th century, adult male Indians were heavily taxed, often being forced into debt peonage. Western Petén and neighbouring Chiapas remained sparsely populated, and the Maya inhabitants avoided contact with the Spanish. San José, on the northwest shore of lake Petén Itzá, is the home of the last surviving speakers of the Itza language. The surname Kowoj still survives, but the Kowoj and Itza have fully merged and no longer exist as separate ethnicities. In modern times there is a history of conflict between San José  and neighbouring San Andrés , and this mutual hostility probably represents ancient hostility between the Itza and the Kowoj.
Question: What caused the population of Petén to fall to just over 3000 people in 1714?
Answer: European-introduced diseases