Article abstract Influenza is caused by infection with an influenza virus which may result in disease of varying severity in humans and a wide variety of animals. Besides the annually recurring seasonal disease outbreaks in humans in the temperate climate zones, influenza is probably best known for its capability to cause pandemics, with the best known and most terrifying example being the ‘Spanish flu’ of 1918. In this review we describe the three major manifestations of influenza A in humans: 1) annually recurring seasonal flu, which in the moderate climate zones is seen in the fall or winter months; 2) avian flu, caused by sporadic zoonotic infections with avian influenza viruses; and 3) pandemic flu or worldwide outbreaks of influenza. In addition, we describe the epidemiology of influenza B and C viruses in humans.