The Nipah virus is on the World Health Organization's short list of diseases that have pandemic potential and therefore pose the greatest public health risk. With a fatality rate at about 70%, it is one of the most deadly respiratory diseases health officials have ever seen. But as regular outbreaks began in the early 2000s in Bangladesh, researchers were left scratching their heads. Initially, the cause of the outbreaks was unknown to them. But once they identified the virus, a second, urgent question arose: How was the virus jumping from bats into humans?This episode is part of the series, Hidden Viruses: How Pandemics Really Begin.