After a few years of adaptation and infection, Nipah became virulent enough to kill both their pig and human hosts. That was probably why the outbreak in Perak and Negri Sembilan took off with such severity in late 1998. That and because pigs were involved. Prof Lam believes that pigs were a key link in Nipah infection of humans. Dr Aziz agrees.
Pigs act as mixing vessels.
Other animals like polo horses, cats, dogs and rats were infected but they died without spreading the virus. The pigs in turn, secreted the virus in their respiratory droplets and urine. Close contact with pigs is now confirmed as the main source of infection. All cases of infection, including those in an abattoir in Singapore and a laboratory in Ipoh involved pig contact. Since JE was spread by mosquito bites, laboratory workers probably didn't expect to be infected through animal tissue.