An outbreak of the deadly Nipah virus in India's West Bengal state has sparked concern in parts of Asia, with some tightening screening measures at airports.

Thailand has started screening passengers at three airports that receive flights from West Bengal. Nepal has also begun screening arrivals at Kathmandu airport and other land border points with India.

Five healthcare workers in West Bengal were infected by the virus early this month, one of whom is in critical condition. Some 110 people who were in contact with them have been quarantined.

The virus can spread from animals to humans. It has a high death range – ranging from 40% to 75% – as there is no vaccine or medicine to treat it.

What is the Nipah virus and what are its symptoms?

The Nipah virus can be transmitted from animals, like pigs and fruit bats, to humans. It can also spread person-to-person through contaminated food.

The World Health Organization has described Nipah in its top ten priority diseases because of its potential to trigger an epidemic.

The incubation period ranges from four to 14 days.

People who contract the virus show a wide range of symptoms, or sometimes, none at all.

Initial symptoms may include fever, headaches, muscle pain, vomiting, and sore throat. In severe cases, patients may experience drowsiness, altered consciousness, and pneumonia.

Where were the past outbreaks?

The first recognized Nipah outbreak was in 1998 among pig farmers in Malaysia and later spread to neighboring Singapore. The virus got its name from the village where it was first discovered.

More than 100 people were killed and a million pigs culled in an effort to contain the virus

Bangladesh has borne the brunt in recent years, with more than 100 people dying of Nipah since 2001.

What is happening now?

At least five confirmed cases were reported as of last week, all linked to a private hospital in Barasat. Two nurses are being treated in critical condition.

No cases have yet been reported outside India, but several countries are stepping up precautions. On Sunday, Thailand began screening passengers from West Bengal flights. Nepal is also screening arrivals at its airport and land borders.