Influenza is an acute viral respiratory disease that is often characterized by fever, headache, myalgia, sore throat and cough. Influenza infection is transmitted through respiratory droplets or contact and is clinically indistinguishable from other respiratory viral diseases without laboratory confirmation.
Human influenza is caused by the influenza viruses which are classified into three main types: influenza A, B and C. Influenza A and B viruses can cause epidemic disease in humans and type C viruses usually cause a mild, cold-like illness. The natural reservoir of influenza viruses are aquatic birds and influenza can cause disease in a range of mammalian species including pigs, seals and horses.
Influenza viruses can spread rapidly around the world causing seasonal epidemics that can result in significant morbidity and mortality. In temperate climates influenza is seasonal, typically occurring every year in late autumn or winter. In tropical and subtropical regions the existence of seasonal patterns of influenza transmission are unknown and sporadic cases of influenza can occur year-round. Data describing seasonality, epidemiology, transmission patterns and disease burden from human influenza in the African Region is limited. Influenza vaccines are produced every year against seasonal influenza and offer protection for vulnerable groups in the community.
Pandemic influenza is caused by a new or novel influenza that is introduced into a population where few people are immune. In the last hundred years, four global pandemics have occurred. The 1918 pandemic (influenza A/H1N1) which infected an estimated 500 million and killed 50-100 million people worldwide has been the most devastating pandemic to date with more recent pandemics; 1957 Asian Flu pandemic (influenza A/H2N2), 1968 Hong Kong flu pandemic (influenza A/H3N2) and the 2009 (influenza A[H1N1]pdm09) resulting in far fewer deaths.
Avian influenza can also infect humans and the first cases of human infection with a highly pathogenic avian influenza (A/H5N1) were reported in 1997 in Hong Kong Special Administrative Region, China. Since 2003, sixteen countries, representing all WHO regions have reported human infections of A/H5N1 with a total of 840 cases and 447 deaths recorded. In addition to A/H5N1 an increasing number of other avian influenza A viruses are being recognised as causing sporadic infections in humans. Human disease caused by some avian viruses can be mild however certain avian viruses can cause severe disease and mortality. A number of avian influenza outbreaks have been reported in the African region resulting in the large scale death or destruction of backyard and commercial poultry, however few human cases have been reported.