Sometimes the answer to a health mystery lies in a swig of booze.

In Bangladesh in recent years, there have been repeated mini-outbreaks of a disease called Nipah virus – three people here, four there.

Some people develop no symptoms. But in others, the virus can progress from a fever to fatal brain inflammation within a week.

A few years ago, epidemiologists figured out that people were likely getting Nipah from drinking raw date palm sap, a sweet drink popular in the winter, when the sap is easy to tap from trees pierced with a spigot.

Bats also like the sap and were caught on infrared camera licking streams of it dripping into collection pots. "So you get bat saliva into the stream," says Dr. Stephen Luby, an epidemiologist at Stanford University who worked on solving this initial mystery few years ago. If the bats were shedding virus in their saliva, then a person could get the virus from drinking contaminated raw sap. "We also have some pretty dramatic photos of bats peeing into the date palm sap jars," says Luby, another possible contamination route.

After that revelation, the government of Bangladesh launched a don't-drink-sap campaign.

That should have been a blow to the virus' ability to pop up periodically. But 14 cases were recorded last year; eight of the patients died. Researchers were puzzled. The patients' friends and relatives said they didn't drink raw sap and hadn't come into contact with sick people or animals. How had these people caught the virus? Were they all infected the same way?

The answer, it turns out, lies in a fact that the patients weren't sharing with disease detectives. Even though alcohol consumption is restricted in Bangladesh because of Muslim law (it's heavily taxed and available to foreigners at fancy hotels and certain bars), some men drink date palm wine on a regular basis.

People make tari, or palm wine, by collecting sap overnight from trees pierced with a spigot, the same way they collect raw sap. They let the sugary liquid sit for a few days fermenting in a jar, and the result is a drink with about the same alcohol content as a beer (about 5 to 8 percent) that sells for the equivalent of $2 a gallon. The main customers are men, mostly bus drivers, day laborers, rickshaw pullers and farmers.

For some people it's a daily habit to drink with friends at the end of the day, like having a glass of wine with dinner, says Dr. Emily Gurley, an infectious disease epidemiologist who directs a team of disease detectives based in Dhaka, Bangladesh. But, she says, drinking alcohol is "stigmatized within the community. Relatives might not want to talk about it because it might make people perceive your family poorly or perceive your loved one who just died badly." (Taboos can get in the way of disease detection in the U.S., too – like in a salmonella outbreak in the 1980s that, after interviews failed to dredge up a common link, was finally traced to contaminated marijuana).

So the group had to tread carefully during interviews with surviving patients, or for those who didn't make it, their relatives and neighbors. "We didn't go specifically asking about fermented sap, but we did ask, 'How did they know each other? What did they do together?' " says Gurley. They also relied heavily on locally-trained anthropologists, who knew much better than the physicians and epidemiologists how to gain trust.

The research group — consisting of the U.S. Centers for Disease Control and Prevention, its equivalent in Bangladesh, a group called EcoHealth Alliance and the health research organization where Gurley works — found that of the 14 cases, the eight men who died of Nipah virus all drank tari every evening. They got fevers, which progressed sometimes to seizures, confusion and comas before death. The six women, who all survived, had likely caught the virus while caring for the sick men.

Even though the number of Nipah cases has been fairly small — between 2001 and 2014 there were 30-plus small outbreaks in Bangladesh and India — scientists are concerned.

"This is a disease that goes from animals to people and then it can go from person to person," says Luby. "These are the bugs that we worry about." Because if a strain of Nipah crops up that's capable of jumping between people more efficiently, it could be a real problem.

The usual host for the virus is the jumbo-size fruit bats in the Pteropus family, also called flying foxes. In Malaysia, it jumped from bats to pigs to people. But in Bangladesh, the virus has shown itself capable of hopping between people. In one case, it jumped four times after first infecting someone through sap.

"I think we are in watch-and-wait mode with this virus," says Eddie Holmes, a virologist at the University of Sydney in Australia. As he wrote this week in the journal PNAS, there are a few things a virus would need in order to have pandemic potential. Nipah is lacking a few.

First, it's not very good at passing from human to human. Second, Holmes explains, "Nipah is associated with a very high mortality rate in humans and as a general rule — and this is really only a generality — viruses with such a high virulence usually do not lead to pandemics." From the virus perspective, it's not a very good strategy to kill off everyone who is infected so quickly that they don't have time to spread it to others first.

But the virus could evolve the ability to move more easily between people.

"The really important outbreaks to watch are not the 'accidental' exposures to bats, but those where there has been human-to-human transmission: the more of these that occur, and the longer they occur, then the more Nipah virus will be able to adapt fully to humans. This is clearly the concern," says Holmes.

In the meantime, Luby, Gurley and colleagues will continue tracking the clusters of cases that are certain to pop up across the country.

Luckily, there is an easy way to keep people from getting it: bamboo covers to protect the containers of sap from bats at night.

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