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General Information about Influenza

Flu VirusInfluenza, commonly called "the flu," is an infection of the respiratory tract caused by the influenza virus. Compared with most viral respiratory infections, such as the common cold, influenza infection often causes a more severe illness. Typical influenza illness includes fever (usually 100 degrees F to 103 degrees F in adults and often even higher in children) and respiratory symptoms, such as cough, sore throat, runny or stuffy nose, as well as headache, muscle aches and extreme fatigue. Although nausea, vomiting and diarrhea can sometimes accompany influenza infection, especially in children, these symptoms are rarely the primary symptoms. The term "stomach flu" is a misnomer that is sometimes used to describe gastrointestinal illnesses caused by organisms other than influenza viruses.

Most people who get the flu recover completely in 1 to 2 weeks, but some people develop serious and potentially life-threatening medical complications, such as pneumonia. Over the past decade, influenza and pneumonia have been associated with an average of 3,500 deaths a year in Illinois. Since 1992, the highest number of flu and pneumonia deaths was the 4,021 recorded in 1993. Flu-related complications can occur at any age, but the elderly and people with chronic health problems are much more likely to develop serious complications after influenza infection than are young, healthier people. During most flu seasons, which typically run from October through May, between 10 percent and 20 percent of the population is infected with influenza viruses. More than 200,000 people are hospitalized from flu complications each year in the U.S.

Influenza Viruses

Influenza viruses are divided into three types, designated A, B and C. Influenza types A and B are responsible for epidemics of respiratory illness that occur almost every winter and are often associated with increased rates for hospitalization and death. Influenza type C differs from types A and B in some important ways. Type C infection usually causes either a very mild respiratory illness or no symptoms at all. It does not cause epidemics and does not have the severe public health impact that influenza types A and B do. Efforts to control the impact of influenza are aimed at types A and B.

Flu vaccines are designed to protect against the three influenza viruses that experts predict will be the most likely to cause illness during the upcoming season. Each influenza season, this includes an influenza B virus, and two influenza A viruses that represent the three virus subtypes circulating most commonly among people at that time.

The 2013-2014 trivalent influenza vaccine is made from the following three viruses:

  • an A/California/7/2009 (H1N1)pdm09-like virus;
  • an A(H3N2) virus antigenically like the cell-propagated prototype virus A/Victoria/361/2011; and
  • a B/Massachusetts/2/2012-like virus.

It is recommended that quadrivalent vaccines containing two influenza B viruses contain the above three viruses and a B/Brisbane/60/2008-like virus.

Learn about Who Needs A Flu Vaccine.

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