Influenza viruses are the causative agents of the influenza or flu.These are RNA-viruses of the viral family the orthomyxoviridae. The genome of the influenza viruses is segmented and comprises eight separate antisense RNA strands, each of which codes for one specific protein. Together with the nucleoprotein, they form the helical nucleocapsid. The nucleocapsid itself is embedded in a protein. The virus is enclosed by an envelope made of cell membrane lipids with viral protein inclusions (hemagglutinin and neuraminidase, responsible for infectivity and viral progeny release) (see figure).Influenza viruses are genetically variable. Slight antigenic changes are the general rule (antigenic drift) and are explained by selection of point mutants in the hemagglutinin under immunological pressure. More profound changes (antigenic shifts) explain the periodic occurrence of influenza epidemics and pandemics.The aerogenically transmitted influenza viruses normally replicate in the mucosa of the nasopharynx, resulting in a pharyngitis or at most a tracheobronchitis, after an incubation period of 24-72 hours. Pulmonary dissemination of the infection can result from an upper respiratory infection or manifest without one, whereby the prognosis in the latter case is less favourable. Pneumonia caused solely by the influenza-virus is rare. As a rule, superinfections with staphylococci, streptococci, pneumococci or Haemophilus bacteria are responsible. These infections, which used to be the normal cause of an influenza death, can be controlled with antibiotics.
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