An outbreak of human encephalitis in February 2004 in Bangladesh has so far caused 17 deaths. The US Centers for Disease Control (Atlanta, GA) has confirmed that these and 22 nonfatal cases were caused by a virus of the genus Henipavirus. All past outbreaks of these viruses have been linked to flying foxes (genus Pteropus). The virus behind the latest Bangladesh outbreak may be a new member of this genus, as epidemiologically it behaves differently than other known species of Henipavirus. The World Health Organization is still investigating the Bangladesh events, but the assumption is that flying foxes are the natural host. The worst Henipavirus outbreak to date was of Nipahvirus, which killed 105 people in Malaysia between 1998 and 1999, and involved pigs as amplifier hosts. Spillovers of related viruses have occurred in Australia, India, and previously in Bangladesh.