Influenza

Influenza, also known as the ‘flu,’ is a contagious disease caused by infection with the influenza virus. It is a common disease, with annual attack rates of 10% to 30% worldwide each year, with most cases in the northern hemisphere occurring during the ‘flu season’ of December through March. Flu is characterized by fever, by respiratory symptoms, including rhinorrhea, cough, and sore throat, and sometimes by myalgia and headache. In children and infants, gastrointestinal symptoms such as vomiting and diarrhea occur in 50% of cases, but such symptoms are rare in adult cases. The course of influenza is usually self-limiting (i.e., resolves without medical intervention) and lasts [Page 534]3 to 5 days, but serious complications, including pneumonia, may develop that prolong the illness and may prove fatal.

Influenza is spread through the respiratory secretions of infected persons, primarily through airborne secretions spread by coughing, sneezing, and talking or by direct (e.g., kissing) or secondary contact (e.g., touching a surface touched by an infected person and then touching one's nose). Influenza has an incubation period of 1 to 4 days, and infected persons can transmit the virus from 1 day before the onset of illness through the fourth or fifth day of infection. The most common strains of influenza are Types A and B, which cause the annual epidemics and against which the flu vaccine offers protection; Type C causes mild illness, is not implicated in the annual epidemics, and is not included in the flu vaccine. Strains of Type A influenza virus are classified by two proteins found on the surface of the virus, hemagglutinin (H), and neuraminidase (N); Type β virus is not divided into subtypes. New strains of flu are constantly evolving, and the need to identify each new strain led to development of the standard five-part nomenclature, which identifies the virus type, the site of first identification, the strain number, the year of isolation, and the subtype (for Type A virus). For instance, ‘A California/7/200(H3N2)’ refers to a Type A virus first isolated in California in 2004 as Laboratory Strain 7, with subtype H3N2. Popular names of flu viruses generally refer to the geographical region where the outbreak began or was first reported, such as the ‘Spanish Flu’ or the ‘Hong Kong Flu,’ and strains are often referred to by their subtype as well, for instance the H5N1 strain of avian flu.

Although there have been no flu pandemics since the 1960s, flu is still a serious health concern that annually causes many cases of disease and death. The Centers for Disease Control and Prevention (CDC) reports that 5% to 20% of the United States population contracts the flu in an average year and that more than 200,000 are hospitalized and more than 35,000 will die from complications of the flu. Assessing the burden of the flu worldwide is more difficult, but the World Health Organization (WHO) estimates that 3 to 5 million cases of severe illness are caused by the flu each year and between 250,000 and 500,000 deaths.

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