Influenza: old and new threats.

Influenza remains an important disease in humans and animals. In contrast to measles, smallpox and poliomyelitis, influenza is caused by viruses that undergo continuous antigenic change and that possess an animal reservoir. Thus, new epidemics and pandemics are likely to occur in the future, and eradication of the disease will be difficult to achieve. Although it is not clear whether a new pandemic is imminent, it would be prudent to take into account the lessons we have learned from studying different human and animal influenza viruses. Specifically, reconstruction of the genes of the 1918 pandemic virus and studies on their contribution to virulence will be important steps toward understanding the biological capabilities of this lethal virus. Increasing the availability of new antiviral drugs and developing superior vaccines will provide us with better approaches to control influenza and to have a positive impact on disease load. A concern is that the imposition of new rules for working with infectious influenza viruses under high security and high containment conditions will stifle scientific progress. The complex questions of what makes an influenza virus transmissible from one human to another and from one species to another, as well as how the immune system interacts with the virus, will require the active collaboration and unencumbered work of many scientific groups.