During this time, the infection spread to other farms in the Tambun area, probably via movement of infected pigs from the index farm, which supplied gilts and piglets to smaller operations in the vicinity. Subsequent pig movement (including 'fire sales' of pigs in reaction to the cluster of human cases in September-November 1998) allowed the virus to spread throughout Perak and eventually south to Negeri Sembilan and Selangor [15] . Because there was no evidence of transmission between humans [35] , the pattern of human infection necessarily followed the spread of the virus in pigs, explaining why human cases were much more widespread in 1998 -1999 than in 1997. In addition, the high density of pig farms in many parts of the south (electronic supplementary material) may have allowed respiratory transmission of the virus between adjacent farms without physical movement of infected pigs, contributing to the rapid spread of infection and high number of human cases in the south. On the other hand, these areas were buffered from phase I emergence via bat-to-pig transmission by the absence of flying foxes in the area [33] ( figure 4) and, possibly, a lesser degree of overlap between fruit and pig production (electronic supplementary material).