Nipah virus is a zoonotic virus in the Paramyxoviridae family. An outbreak of encephalitis in Malaysia in 1998 led to its discovery. Since then, Nipah virus outbreaks have been recorded in Asia, in particular Bangladesh and India, almost yearly.

Fruit bats (Pteropus genus), also known as flying foxes, are the natural hosts. Pigs, dogs, cats, horses, goats, and humans can acquire the virus through exposure to food contaminated with the urine or saliva of bats.

During the epidemic in Malaysia, which also affected Singapore, pigs were the secondary hosts. Infection was spread to other pigs and to humans through respiratory droplets. The Malaysian outbreak resulted in nearly 300 human cases and more than 100 deaths. Infections in humans were most often described among people who worked with pigs, either as handlers or in abattoirs (slaughterhouses). Human-to-human and dog-to-human transmissions were also described. A study showed that increasing age and respiratory symptoms were indicators of increased infectivity among humans.

In outbreaks in Bangladesh and India, studies have linked human infection to consumption of fruits or fruit products, such as raw date palm sap (juice), contaminated with urine or saliva from infected fruit bats. Human-to-human transmission among family and caregivers of infected patients and in health care settings has been reported.

The incubation period of Nipah virus infection can range widely but is commonly between 4 and 14 days. Asymptomatic or subclinical infection occurs in 8%-15% of cases. In symptomatic patients, the initial symptoms include sudden onset of fever, nausea, vomiting, headache, and drowsiness. Respiratory symptoms are present in a minority of cases. Patients often have depressed levels of consciousness and brain stem dysfunction. The condition may progress rapidly to encephalitis with myoclonus, areflexia, hypotonia, tremor, seizures, ataxia, ptosis, altered mental state, convulsions, and coma. Hypertension and tachycardia may also be present.

The mortality rate of Nipah virus infection is 40%-75%, with a quarter of all survivors suffering permanent neurologic sequelae.