Nipah virus infection is a zoonotic illness that is transmitted to people from animals, and can also be transmitted through contaminated food or directly from person-to-person. In infected people, it causes a range of illnesses from asymptomatic (subclinical) infection to acute respiratory illness and fatal encephalitis. The virus can also cause severe disease in animals such as pigs, resulting in significant economic losses for farmers.

Although Nipah virus has caused only a few known outbreaks in Asia, it infects a wide range of animals and causes severe disease and death in people.

During the first recognized outbreak in Malaysia, which also affected Singapore, most human infections resulted from direct contact with sick pigs or their contaminated tissues. Transmission is thought to have occurred via unprotected exposure to secretions from the pigs, or unprotected contact with the tissue of a sick animal.

In subsequent outbreaks in Bangladesh and India, consumption of fruits or fruit products (such as raw date palm juice) contaminated with urine or saliva from infected fruit bats was the most likely source of infection.

Human-to-human transmission of Nipah virus has also been reported among family and care givers of infected patients.

Human infections range from asymptomatic infection to acute respiratory infection, seizures and fatal encephalitis. Infected people initially develop symptoms that include fever, headaches, myalgia, vomiting and sore throat. This can be followed by dizziness, drowsiness, altered consciousness, and neurological signs that indicate acute encephalitis. Some people can also experience atypical pneumonia and severe respiratory problems, including acute respiratory distress. Encephalitis and seizures occur in severe cases, progressing to coma within 24 to 48 hours.

The incubation period is from 4 to 14 days but an incubation period as long  as 45 days has been reported.

Most people make a full recovery, although some are left with residual neurological conditions after acute encephalitis. Some cases of relapse have been reported. 

The case fatality rate of Nipah virus infection is estimated at 40–75% but can vary by outbreak depending on surveillance and clinical management in affected areas. 

There are currently no drugs or vaccines that specifically target Nipah virus infection. WHO has identified Nipah as a priority disease for the WHO Research and Development Blueprint.

Intensive supportive care is recommended to treat severe respiratory and neurologic complications.