Influenza

Influenza, commonly known as the flu, is an infectious disease of birds and mammals caused by an RNA virus, influenza A or B virus, of the family Orthomyxoviridae. In humans, common symptoms of influenza infection are fever, sore throat, muscle pain, severe headache, coughing, and weakness and fatigue. In more serious cases, influenza causes pneumonia, which can be fatal, particularly in young children and the elderly. Influenza A viruses are further classified into subtypes based on the antigenic properties of their surface spike glycoproteins, the hemagglutinin (HA) and neuraminidase (NA). There are 16 known HA subtypes (H1-H16) and nine NA subtypes (N1-N9). Influenza virus is transmitted from infected mammals through the air by coughs or sneezes, creating aerosols containing the virus, and from infected birds, particularly aquatic birds, through their droppings and contamination of lake water. Influenza spreads around the world in seasonal epidemics, killing millions of people in pandemic years and hundreds of thousands in nonpandemic years. Three influenza pandemics occurred in the twentieth century (1918-19, 1957, and 1968). Each of these pandemics was caused by the appearance of a new strain of the virus in humans. These new strains resulted from the direct transfer of an avian virus to humans (1918) or the mixing of genes between human viruses and avian viruses. A highly pathogenic avian virus first killed humans in Asia in 1997. This deadly avian strain (H5N1) has posed the greatest risk for a new influenza pandemic. However, this virus has not yet mutated to spread easily among humans. Vaccinations against influenza are given to humans. The most common human vaccine is inactivated virus. The vaccine is reformulated each year because influenza virus accumulates point mutations in its surface glycoproteins (antigenic drift) and different strains become dominant. Antiviral drugs can be used to treat influenza, with NA inhibitors (zanamivir and osteltamivir) being most effective.