Above our heads, hundreds of fruit bats are coming home to roost. Their translucent wings stretch wide as they maneuver between careening bodies and twisted tree limbs before braking hard, grasping with outstretched feet, and awkwardly hanging themselves upside down for the day. As disorderly as the bats’ morning approach may seem, the real scrum begins when they land. Someone’s always too close, or hanging in a prime patch of sun or shade. Often the only way to regain one’s rightful place in the roost is with a warning screech or a swift nip or wingslap.

Fortunately for these bats, the antagonism remains within the colony. This group of Indian flying foxes (Pteropus medius) has been lucky enough to take up residence in an enormous sacred fig tree (Ficus religiosa), which sits at the center of a local Hindu temple. Though the fig’s twisted trunk brings to mind the menacing trees from The Wizard of Oz, the temple’s low, deteriorating walls demarcate a peaceful place in which the tree and its many inhabitants can live out their existence free from harassment.

While the temple offers an extra level of protection, people in Bangladesh are generally resigned to living among bats. “They’re everywhere,” says Gurley. “There’s hardly any village, among the hundreds we’ve visited, that doesn’t have a bat roost.”

Driven by a lack of primary forest and reliable food resources across much of their native range, the bats likewise seem resigned to living near people. Just like their relatives in Malaysia, bats in this region frequently take advantage of cultivated crops such as mangoes and figs to make up for the native fruit and nectar missing from their diets. Many have also developed a taste for date palm sap.

In 2004, when Gurley first began studying Nipah in earnest, she and her team of investigators quickly zeroed in on that bat-sap connection. They asked Nipah survivors and loved ones of the deceased about direct contact with bats, about tree climbing, about living near bat roosts and eating fruit that had fallen from trees. They also asked about consumption of date palm sap. Of all the activities the team explored, sap drinking was one of the most consistent among those who had fallen ill.

The clearest evidence of sap contamination, though, came in 2010 when researchers set up infrared cameras trained on the collection pots that gachhis had hung on date palm trees. The results of this study couldn’t have been more telling. Over 20 nights of observation, the cameras captured 132 visits from fruit bats on 14 separate nights. Bats could be seen lapping up the sweet sap oozing from the sections of tree trunk where the gachhis had shaved the bark away. In some instances, the images captured bats in the act of relieving themselves directly above the collection pots. The contamination was unintentional, of course, but the possible route of transmission was undeniable.

Among mammals, bats rank number one in terms of their role in spreading zoonotic diseases. Scientists have linked them to at least a dozen human afflictions, including four of the eight diseases the WHO has prioritized. Exactly what makes bats such good hosts for disease-causing agents—or if in fact they are particularly suited to carrying and transmitting disease—has been poorly understood for decades. But new research suggests that the answers may have something to do with flight.