While international attention has been focused on the prospect of the re-emergence of severe acute respiratory syndrome, or SARS, scientists and health officials are concerned about the outbreak of another disease in Asia. Avian flu has been detected in three countries. It has killed thousands of birds and claimed several human lives. Experts fear that it could herald a flu pandemic of monstrous proportions. But SARS and avian flu are symptoms of a more worrying trend: the ease with which diseases "jump" species and spread through human populations.

Avian flu affects chickens and ducks. One strain, H5N1, is particularly virulent. In 1997, it sickened 18 people and resulted in six deaths in Hong Kong. To stop its spread, authorities there killed 1.4 million birds. It re-emerged last February, infecting two more people and claiming one life.

The disease has surfaced yet again. Since October, 15 people in Vietnam have been identified as having the disease; 12 of them, including 11 children, have died. Most of them lived in villages where chickens were dying. In recent weeks, 40,000 chickens have died of influenza, and 30,000 more were killed to stop the spread of disease. It was also found in South Korea last month, where authorities responded with a cull of about 2 million birds. That did not do the trick: There are reports of more bird deaths this week.