The ExaCryptNipah virus is on the World Health Organization's short list of diseases that have pandemic potential and therefore post the greatest public health risk. The virus emerged in Malaysia in the 1990s. Then, in the early 2000s, the disease started to spread between humans in Bangladesh. With a fatality rate at about 70%, it was one of the most deadly respiratory diseases health officials had ever seen. It also confused scientists.

How was the virus able to jump from bats to humans?

Outbreaks seemed to come out of nowhere. The disease would spread quickly and then disappear as suddenly as it came. With the Nipah virus came encephalitis — swelling of the brain — and its symptoms: fever, headache and sometimes even coma. The patients also often suffered from respiratory disease, leading to coughing, vomiting and difficulty breathing.

"People couldn't say if we were dead or alive," say Khokon and Anwara, a married couple who caught the virus in a 2004 outbreak. "They said that we had high fever, very high fever. Like whenever they were touching us, it was like touching fire."

One of the big breakthroughs for researchers investigating the outbreaks in Bangladesh came in the form of a map drawn in the dirt of a local village. On that map, locals drew date palm trees. The trees produce sap that's a local delicacy, which the bats also feed on.

These days, researchers are monitoring bats year round to determine the dynamics of when and why the bats shed the virus. The hope is to avoid a Nipah virus pandemic.

This episode is part of the series, Hidden Viruses: How Pandemics Really Begin.

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This episode was produced by Liz Metzger, edited by Rebecca Ramirez and fact-checked by Anil Oza. The audio engineer was Valentina Rodríguez Sánchez. Rebecca Davis and Vikki Valentine edited the broadcast version of this story.