The influenza virus is unique in its ability to mutate and adapt, with new variants arising every year in animal populations around the world. This has been the chief obstacle to creating a once-in-a-lifetime vaccine like those used for polio or diptheria. The flu vaccines to date have to be reconfigured and re-administered every year after the predominant human strains become apparent.

This may be about to change. According to an article in Science Daily, The British-American biotech company Acambis has successfully tested a "universal" flu vaccine in humans. It is not exactly universal: it combats all "A" strains of the flu, which are responsible for approximately 2/3 of the cases every year. However, it is a major step forward, and would provide protection against all the "pandemic" strains which have caused so many deaths in the past, and avian flu, which has the scientific community so worried.