Nipah virus is a zoonotic virus in the Paramyxoviridae family. An outbreak of encephalitis in Malaysia in 1998 led to its discovery. Bats of the Pteropus genus, also known as flying foxes, are the natural hosts. Pigs, dogs, cats, horses, goats, and likely humans can acquire the virus through exposure to food contaminated with the urine or saliva of bats. During the epidemic in Malaysia, pigs were the secondary hosts. Infection was spread to other pigs and to humans through respiratory droplets. Human-to-human and dog-to-human transmissions have also been described. Infection among pigs is mostly a respiratory illness; it is highly contagious among them but associated with low mortality. Infections in humans are most often described among people who work with pigs, either as handlers or in abattoirs (slaughterhouses). A study has shown increasing age and respiratory symptoms are indicators of increased infectivity among humans. Reducing exposure to body fluids is key.

The incubation period of Nipah virus infection can range widely but is commonly between 4-14 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. Nipah virus infection has only been seen in Asia and was first noted in Malaysia.

Abattoir workers, pig farmers, and travelers to Asia are at greater risk for contracting Nipah virus infection.