Long before sars was an international epidemic—when it was still a mysterious disease affecting people in Guangdong Province, doctors around the world were hearing rumors by way of the Internet. Always concerned about the emersion of a new illness, WHO researchers around the world were keeping track of any information they could get about the pneumonia-type illness.

Six Percent

Any new disease causes worry, and there have been a number of killers that have emerged in the past quarter century—from AIDS to Ebola. In addition, WHO doctors around the world have monitored oubreaks of well-known but dangerous diseases such as malaria and cholera. In many cases, the disease can be identified and contained in one area before it spreads out of control. However, the most frightening of all scenarios, say experts, is that a highly contagious disease that is unknown to doctors...