Between September 1998 and April 1999, NiV caused 246 reported cases of febrile encephalitis in humans in peninsular Malaysia and Singapore [7, 8] and an epidemic of respiratory and neurological disease in commercially farmed pigs [9, 15] . Initial human cases were seen in Tambun village and surrounding areas in Perak State, followed by a large epidemic in the southern states of Negeri Sembilan and Selangor and several cases in Singapore (figure 1a). The majority of these cases occurred in pig farmers and abattoir workers [17] . During the course of the outbreak investigation, nine cases of NiV encephalitis were retrospectively diagnosed in the Tambun area with onset dates between January 1997 and May 1998, five of which were associated with the index farm of the 1998 -1999 outbreak in Perak (figure 1a). Six of the nine cases are considered confirmed cases on the basis of serological tests performed in 1999 [16] ; the remaining three are probable cases.