The current influenza epidemic in the us, the avian flu and severe acute respiratory syndrome (sars) in Southeast Asia are infectious diseases that have rapidly evolved, taking national and global health systems by surprise. Such infections emerge in one corner of the globe and hitchhike into new areas, often causing great mortality. At least four pandemics of influenza have occurred in the 20th century. The 1918-1919 "Spanish flu' was the most severe: an estimated 21 million people died. According to recent studies, the explosive spread of the pandemic was enabled by the unique virulence acquired by the virus. Virologists now reveal the 1918 influenza strain underwent subtle alterations that enabled it to bind with deadly efficiency to human cells, while retaining the basic properties of the avian virus from which it evolved.

Such changes in the degree of virulence of flu viruses have caused major concern to virologists. The surface antigens of viruses