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.

Many questions remain, not least of which is, is this episode over? There is still no agreement on how the virus is transmitted. Thus far, pigs, horses, mosquitoes, bats, cats and dogs have been identified as carriers. Identifying the vector is critical. Before a quarantine was put in place, infected pigs sickened 11 workers in a Singapore slaughterhouse; one man died. Authorities believe that only live infected pigs transmit the virus; that means meat should be safe, but here too, no one is certain.

The Malaysian government’s response to the outbreak has come under intense scrutiny, particularly its reluctance to acknowledge that the virus was not a strain of Japanese encephalitis. Malaysia’s delicate ethnic balance between Chinese and Muslims has been blamed for complicating the situation. Fears that tourism would be harmed may have also dampened a high-profile response to the problem.

That six months passed between the first signs of the outbreak and the claim that it was under control is worrisome. That is plenty of time for the virus to spread; there is little reason for alarm since residents of the infected areas — rural pig farmers — are unlikely to travel far. Experts fear that there will be more discoveries of infected animals, however.

The Malaysian virus is not the only such outbreak in recent months. In Singapore, authorities are concerned about a new “superbacteria” that has developed resistance to antibiotics and is especially dangerous to the old and the young. In Hong Kong, there have been two more cases of the bird flu that killed six people two years ago, the cause of which is still unclear. In the Netherlands, an outbreak of Legionnaire’s Disease in February has claimed 20 lives and sickened 231 others. In Zimbabwe, there are cases of soldiers infected with an Ebola-like virus they apparently contracting during their tour of duty fighting in Congo.

Five years ago, a spate of books and movies put infectious diseases in the spotlight. After a bit of handwringing, the issue receded and has, for the most part, retreated to the science pages. Nonetheless, the brief flash of publicity was illustrative. It showed that attempts to deal with deadly infectious diseases were not immune to the normal pathogens in international cooperation: turf fights between institutions, differences in priorities and perceptions among developed and less developed nations, and cultural sensitivities. In other words, combating microbial killers is as much a social and cultural problem as it is medical. That is not reassuring since the viruses themselves are utterly indifferent to such concerns. They are equal opportunity killers and the world seems determined to give them the chance to do their worst.

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