There are three basic types of influenza viruses: A, B, and C. These RNAs are negative-sense molecules, meaning that they must be copied into positive-sense molecules in order to direct the production of proteins. The genomes of all influenza viruses are composed of eight single-stranded RNA segments (Figure 1). The name 'influenza' is derived from the Latin word for 'influence,' and the pathogens that cause this disease are RNA viruses from the family Orthomyxoviridae. Later epidemics occurred in 1957, when the 'Asian' flu killed 70,000 people in the United States, and in 1968, when the 'Hong Kong' strain killed 30,000 Americans (Reid & Tautenberger, 2003). In fact, people under age 65 accounted for 99% of the deaths attributed to this strain, whereas subsequent pandemics claimed many fewer people from this age group. Unlike weaker flu strains that are more of a threat to the elderly, this flu claimed the lives of many young people, including children and young adults. The first, which took place in 1918 and was referred to as 'Spanish' influenza, was the deadliest, claiming an estimated 40 million lives worldwide in less than a year (Palese, 2004).

Since 1900, three of these pandemics have occurred. These strains occur because of the phenomenon known as antigenic shift, in which humans are infected with avian influenza viruses or viruses that contain a combination of genes from human and avian sources. Less frequently, however, new and particularly virulent strains of influenza arise, which cause worldwide pandemics that are accompanied by greatly increased death tolls. These seasonal epidemics are the result of antigenic drift, a phenomenon caused by mutations in two key viral genes due to an error-prone RNA polymerase. Every two or three years, more virulent strains circulate, increasing death tolls by approximately 10,000 to 15,000 individuals.

In the United States, seasonal influenza epidemics typically claim the lives of about 30,000 people each year and cause hospitalization of more than 100,000 (Reid & Tautenberger, 2003).

Visual micro virus