Nipah virus infection is an emerging disease endemic in Southeast Asia. This virus is carried subclinically in fruit bats of the genus Pteropus, a host to which it seems well adapted. Illnesses caused by Nipah virus were first described in 1998- 1999, during widespread outbreaks among pigs and people in Malaysia. The virus had apparently been transmitted from bats to pigs around 1996, and was thereafter maintained in swine populations. It was not detected immediately, as the mortality rate was low and the illness resembled other pig diseases. Nipah virus subsequently spread to pig farmers and abattoir workers in Malaysia and Singapore, causing severe, often fatal, encephalitis in more than 250 people. Some other species, including cats, dogs and goats, were also affected. The Malaysian outbreaks were controlled in both domesticated animals and humans by culling more than one million pigs. In addition, pig farming was permanently banned in some high-risk areas.

While Nipah virus encephalitis has not been documented in Malaysia since that time, human cases have been reported regularly in Bangladesh and a neighboring region of northern India since 2001. Many of these cases seem to be acquired directly from bats by drinking raw date palm sap, a widely consumed local delicacy. The sap is thought to become contaminated when bats visit and drink from unprotected sap collection sites at night. Person-to-person transmission also occurs after close, unprotected contact. How widely Nipah virus circulates in bats is still uncertain; however, viral RNA and seropositive bats have also been identified in areas where no clinical cases have ever been reported. A recent outbreak of neurological disease in horses and humans in the Philippines also appears to have been caused by this virus.