Influenza is a highly contagious, acute respiratory illness that is still a major health problem. Epidemics caused by influenza A viruses occur regularly, often leading to excess mortality in susceptible populations. In addition, influenza A viruses can cause devastating pandemics in humans. An avian influenza A virus originating from Asia and currently circulating among domestic birds in Europe and neighbouring countries makes headlines because of its potential to infect and kill people. If further adaptation to humans occurs, this virus strain might become the origin of a future pandemic. Although influenza viruses belong to the best studied viruses, the host adaptation processes which enable influenza viruses to jump from one species to another are largely unknown. Likewise, the properties required for host-to-host transmission are presently not understood. Moreover, efficient control of influenza virus infections is still not possible. Immunization regimes are continually being confronted with the extreme antigenic variability of influenza A viruses brought about by antigenic drift and shift. It is evident that new approaches and reagents to control influenza are urgently needed.