IN the early hours of March 15, 2003, I was awakened by a telephone call at my home in Geneva from the infectious disease duty officer of the World Health Organization, who had just received a call from the health authorities in Singapore.

He said that a doctor in the city-state who had been treating patients with the unusual respiratory disease that we were monitoring had become ill with the same symptoms while flying back from a medical conference in New York. His plane was due to stop in Frankfurt.

Our first step was to alert the German health authorities and advise them to consider taking the doctor off the plane to reduce exposure to other passengers, and to put him under immediate medical supervision.

What followed marked a turning point in the history of public health as the W.H.O. issued an international appeal that galvanized global cooperation in an unprecedented effort to contain the dangerous disease.