Influenza are simple viruses, composed of a ribonucleic acid genome surrounded by a protein shell. These viruses are responsible for the infectious disease influenza, which affects birds and mammals, inducing symptoms, which include fever, chills, sore throat, headache, muscle pains, coughing, fatigue and general discomfort, according to Centers for Disease Control and Prevention. Three categories of influenza viruses have been identified, each with very different characteristics.
Infection
Transmission of influenza is predominantly through coughs and sneezes, which carry airborne suspensions of the virus, but also through contact with contaminated surfaces and some bodily fluids. In much rarer cases, influenza is transmitted through contact with animal droppings or by eating raw poultry, according to the Health Protection Agency.
Replication
Proliferation of all viruses, including influenza can only occur in living cells. The virus binds to the outside of cells, gaining entry and delivering the genome to a site where the host's biological machinery can be used to duplicate the virus. Influenza A and B have eight ribonucleic acid segments and 11 proteins, whereas influenza C has seven ribonucleic acid segments and nine proteins. After assembly from its constituent parts, the virus is free to exit the host cell and repeat its life cycle, according to an article published in "Vaccine" in September 2008.
Influenza A
Of the three viral types, influenza A is the most virulent human pathogen, responsible for the most severe disease, according to an article published in "Public Health Reports" in 2010. The natural host of influenza A is wild aquatic birds, but occasionally mutations in the virus allow transmission to other species, leading to devastating pandemics such as the 1918 Spanish influenza which killed an estimated 40 to 60 million people worldwide over two years, according to the Health Protection Agency.
Influenza B
Humans and seals are the only two species thought to be infected by influenza B, which is much less common than Influenza A. The low number of species affected by this virus, combined with its slow rate of mutation are thought to be responsible for the lack of influenza B pandemics, according to an article published in December 2001 in "Philos Trans R Soc Lond B Biol Sci."
Influenza C
Affecting humans, pigs and dogs, influenza C is the rarest of the three viral forms, but can cause severe local epidemics, according to key findings published in the "Journal of General Virology" in January 1983. Most adults have been infected with type C influenza, leading to mild upper respiratory tract illness, with lower respiratory tract complications being much rarer.
References
- Centers for Disease Control and Prevention; Seasonal Influenza: The Disease
- Health Protection Agency; Pandemic Influenza
- PubMed; "Public Health Reports"; Influenza: The Once and Future Pandemic; J. Taubenberger, D. Morens; Jan 2010
- PubMed; "Philos Trans R Soc Lond B Biol Sci"; The Evolution of Influenza; A. Hay et al; Dec 2001
- PubMed; "Journal of General Virology"; Isolation of Influenza C Virus from Pigs and Experimental Infection of Pigs with Influenza C Virus; Guo Y et al; Jan 1983
