The virus, which is carried by fruit bats and animals such as pigs, can cause fever and brain inflammation and has a fatality rate of between 40 per cent and 75 per cent. Although it can spread from person to person, transmission is not easy and typically requires prolonged contact with an infected individual.
It more commonly spreads to humans from infected bats, or fruit contaminated by them.
The infections were confirmed in India in late December. Small-scale outbreaks are not unusual and virologists said the risk to the general population remained low. Several vaccines are in development but are still undergoing testing.
"While vigilance is warranted, there is no evidence to suggest a broader public health threat at this stage," said Efstathios Giotis, lecturer in molecular virology at the University of Essex in Britain.
The two people infected in the eastern Indian state of West Bengal in late December were health workers and both are under treatment at a local hospital, a district health officer told Reuters.
Authorities have identified and traced 196 contacts linked to the two cases with none showing symptoms and all testing negative for the virus, the Indian health ministry said in a statement.
"Speculative and incorrect figures regarding Nipah virus disease cases are being circulated," the statement said. "Enhanced surveillance, laboratory testing, and field investigations were undertaken ... which ensured timely containment of the cases."
Reports of the infections put authorities on alert in neighbouring Southeast Asian nations.
Singapore's Communicable Diseases Agency said it will set up temperature screening at its airport for flights arriving from areas affected by the infections in India.
"We are also reaching out to our counterparts in South Asia, to better understand the situation. Work is ongoing to establish a global platform for countries to report genome sequencing of detected cases," the agency said in a statement.
A Hong Kong airport authority spokesperson said it was facilitating enhanced health screening measures enforced by the health department at Hong Kong International Airport, including temperature check at gates for passengers arriving from India.
Thailand earlier this week tightened airport screening measures, with neighbouring Malaysia following suit.
Thailand has assigned designated parking bays for aircraft arriving from areas with Nipah infections, its health ministry said, while passengers must complete health declarations before clearing immigration.
Malaysia's health ministry said it was boosting preparedness through health screening at international ports of entry, particularly for arrivals from countries deemed at risk.
China's disease control authority said no Nipah infections had been detected in the country but there were risks of imported cases, state broadcaster CCTV said.
Nepal, which shares a busy border with India, said it was on "high alert" and had tightened screening for travellers.
Nipah was first identified just over 25 years ago during an outbreak among pig farmers in Malaysia and Singapore, although scientists believe it has circulated in flying foxes, or fruit bats, for thousands of years.
The WHO classifies Nipah as a priority pathogen because there are no licensed vaccines or treatments, its high fatality rate, and the fear that the virus could mutate and become more transmissible.
