Morphology and structure
The influenza virus belongs to the RNA viruses and comes from the family of Orthomyxoviridae. It is usually spherical in shape with a diameter of 80-120 nm. There are 3 distinct types of influenza virus: A, B and C. The genome consists of 8 (type A and B viruses) or 7 (type C virus) segments of RNA joined together. The core of the virus contains single-strand nucleic acid, RNA and soluble nucleoprotein antigen.
Types and subtypes
The variety of structures of the nucleoprotein antigen determines the division of the virus into types A, B and C. Influenza A and B viruses cause seasonal epidemics of disease almost every winter. Influenza type C infections cause a mild respiratory illness and are not thought to cause epidemics. Type A virus is divided into several subtypes depending on the antigenic properties of the surface proteins: neuraminidases (designated as N or NA) and haemagglutinins (designated as H or HA). This virus type infects not only humans, but also other mammals (pigs, horses, mink, seals, whales) and birds. Diseases in humans are most frequently caused by subtypes H1N1, H3N2 and H1N2 and by further variants of these subtypes arising from point mutations. This virus type is responsible for the appearance of recurrent infections that take on the form of an epidemic or pandemic. Type B virus is an aetiological factor of infection only in humans. Influenza caused by this virus type usually has a milder course, although many patients experience the full clinical effect. An increase in the severity of illnesses due to type B virus, periodically also taking the form of an epidemic, is observed on average every 2-3 years. Type C virus causes illness mainly among children. This virus type attacks fairly frequently, but the clinical symptoms are usually mild with the illness taking a subclinical form.