A silent killer has been stalking Malaysia. Since October, over 250 people have been sickened and over 100 have died as a result of a mysterious viral infection. Despite intensive government measures to combat the outbreak, it continues to baffle health investigators. There is uncertainty about the virus' origins, how it is transmitted and even the identity of the bug itself. The killer thus far has been contained, but there is no guarantee that it will not break out: A globe-trotting traveler or a wayward mosquito could spread the virus to other parts of the world. We are reminded once again, that for all of our pretensions, we can still be laid low by the lowest of microbes.

The outbreak began in the pig-farming regions of Malaysia last fall. Hogs would die and shortly thereafter, humans who had direct contact with the animals would sicken; within a week, many of the infected were dead. Malaysian health officials originally identified the killer as a strain of Japanese encephalitis transmitted by mosquitoes. Yet, even after vaccines were administered to men and swine, the disease persisted. In March, more tests revealed that a second virus, a new strain of the "Hendra" virus that had infected horses and caused two human deaths in Australia in 1994 and '95, was the real culprit. There are fears that a third virus is present.

After killing nearly 1 million pigs, closing villages in infected areas and embarking on a mosquito eradication campaign, Malaysian officials claimed to have the virus under control. The World Health Organization echoed that assessment earlier this month, saying the outbreak was virtually over. Unfortunately, the following day Malaysian authorities discovered another reservoir of infected pigs, which they then killed.