Influenza viruses are assumed to be transmitted predominantly by aerosol infection, i.e. relatively large droplets (>5 μm) created particularly while talking, coughing, or sneezing, thus entering the mucosae through contact at small distances. Individual publications, however, have also suggested that transmission might occur by socalled droplet cores which are smaller (<5 μm) and able to remain in the air for longer periods (aerogenic transmission). In addition, transmission can occur by direct contact with virus-contaminated surfaces (e.g. shaking hands) and subsequent mouth-nose contact. After infection the viruses replicate in the nasal and laryngeal mucosae. This replication also affects the lower airways as the infection progresses.A virus can cause deadly infections in humans. Since the first outbreaks of H5N1-related avian influenza in poultry in South-East Asia in 2003, human H5N1 transmissions with high mortality have occurred in 10 countries. It is possible that this virus will adapt better to humans and will then be able to spread among the human population.