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n. a highly contagious virus infection that affects the respiratory system. Types A and B are the forms that most commonly cause outbreaks in humans. The viruses are transmitted by coughing and sneezing. Symptoms commence after an incubation period of 1–4 days and include headache, fever, loss of appetite, weakness, and general aches and pains. They may continue for about a week. With bed rest and aspirin most patients recover, but a few may go on to develop pneumonia, either a primary influenzal viral pneumonia or a secondary bacterial pneumonia. Either of these may lead to death from haemorrhage within the lungs. The main bacterial organisms responsible for secondary infection are Streptococcus pneumoniae, Haemophilus influenzae, and Staphylococcus aureus, against which appropriate antibiotic therapy must be given.

An influenzal infection provides later protection only against the specific strain of virus concerned; the same holds true for immunization. Strains are classified according to the presence of different subtypes of two glycoproteins (antigens) on the viral surface: haemagglutinin (H) and neuraminidase (N). Small changes in the structure of these antigens, which occur frequently in influenza A and B viruses, require the continual development of new vaccines to protect against annual outbreaks of the disease. Major changes in antigenic structure occur much more rarely, when there is genetic recombination between strains that can infect more than one species (most strains of the virus are highly species-specific). However, when it does occur, it could result in the development of hybrid strains causing new forms of influenza that are difficult to contain; the pandemic of 1918–19 is thought to have arisen in this way (see also avian influenza, swine influenza).

Subjects: Medicine and Health — Science and Mathematics.

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