Scientists theorize that the instigating Ebola cases in the most recent epidemic were acquired from a bat colony in a hollowed out tree near a village in Guinea, underscoring the dangers that can come from human and animal environments continuing to overlap.

In the case of the Zika virus epidemic, scientists suspect that rising global temperatures contributed to the spread of the Aedes aegypti mosquito that transmits the disease. These mosquitos, which also transmit West Nile virus, dengue fever and chikungunya, are highly adapted to human environments. Higher temperatures can also expand mosquitos’ flying range, shorten the incubation period for viruses and increase the female mosquito’s biting rate.

And while rain helps mosquitos lay eggs, drought also appears to help them thrive.

Rising temperatures that trigger drought, for example, are the primary climactic driver of the potentially fatal West Nile Virus throughout the U.S., according to a study conducted by disease ecology researcher Sara Paull of the University of California, Santa Cruz. West Nile virus cycles between horses and birds and can be passed on to human beings via mosquitos that have bitten infected animals. Paull found that drought conditions increase transmission of the virus from mosquitos to human beings, for reasons that are still unclear.

“We were able to model the effects of climate change on West Nile virus risk and it looks like the number of human West Nile virus neuroinvasive cases could double or increase pretty substantially if we have higher levels of drought in the future,” she said.

In addition to mosquito-friendly climates, human beings’ globetrotting travel patterns also give viruses a boost. In the U.S., local Zika outbreaks remained confined to small neighborhoods in Florida and Texas, perhaps because of the proliferation of air conditioning units and well-screened windows and doors that keep mosquitos out, but the virus managed to spread to more than 61 countries and territories around the world ― much farther than the range of any typical mosquito.