British health officials are maintaining close surveillance of a Nipah virus outbreak currently unfolding in West Bengal, India, where two cases of the potentially fatal infection have been confirmed. Nipah virus causes severe brain swelling and carries what the UKHSA describes as "a very high mortality rate". The defining characteristic of Nipah infection is encephalitis, an inflammation of the brain, which typically manifests three to 21 days after initial symptoms appear. "This is the hallmark of Nipah infection and is associated with a very high mortality rate," the UKHSA stated. Nipah virus was first identified in 1999 when it emerged among pig farmers in Malaysia and Singapore.