Mayan Rulers would command their followers to wage a war for some materials goods.
An insect from the "plant hopper" family has been linked to spreading a virus. Monkeys are known carriers of yellow fever it can be transmitted to bats and mosquitos. The howler monkeys couldn't have gotten infected. It originated in Africa not until the slave trade in the 1500's
Slash and burn agriculture is not sustainable and will only be effective for a short time. The technique will sustain for 3 - 5 years. To regain fertility, the land will be left fallow.
From about 660 to 1,000 were characterized by repeated and at time, extreme drought. 1020 and 1100 came Maya crop failure, famine, mass migration, and death.
With a drought a disease could've spread since they didn't gave water to get cleansed with. A famine could've started by no water for their crops to grow. They could also have had warfare with other clans to retrieve water from their opponents shelter or terrance.