Lesson 7: Influenza
Organism There are three types of influenza viruses A, B and C with H antigen (15 subtypes) and N antigen (9 subtypes), so that the virus is designated as H1N1, H1N2, . . . , H3N2. In addition, the site of isolation, culture number and year of isolation are used (e.g. A/Beijing/262/95) (H1N1). So
far a major antigenic shift to H4 or N3 in human infection has not yet occurred. Antigenic drift in both A and B viruses, producing new strains occurs at infrequent intervals and is responsible for most epidemics.
Influenza presents with fever, malaise, muscle aches and upper respiratory symptoms of sudden onset. There is initially a dry cough, which can sometimes be severe and often leads to secondary infection, with the production of sputum. It is a serious infection in the elderly with high death rates. When a major antigenic shift occurs as it did in 1918, all ages are susceptible and the number of deaths can be enormous (an estimated 50 million).