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Flu Basics

Author(s): Sonia Rahmati Clayton, PhD, and Nancy Moreno, PhD

Flu: An Introduction

Influenza, commonly known as flu, is a contagious disease caused by the influenza type A, B, and C viruses. Type A, the most common and severe form in humans, also infects other animals, such as ducks, chickens, pigs, whales, horses, and seals. Type B circulates only among humans. Type C is believed to cause only mild respiratory illness and never has been associated with a large epidemic. Influenza type A is believed to be responsible for global flu outbreaks in 1918, 1957 and 1968.

In humans, influenza virus attacks the respiratory tract (nose, throat and lungs). Most people who contract influenza recover within a couple of weeks. However, life threatening conditions develop in some people. Annually, approximately 200,000 persons are hospitalized in the US due to flu (Thompson, et al., 2004; CDC, 2004). In addition, flu causes an average of 36,000 deaths per year.  As reported by Thompson, et al. (2004), people 85 years or older had the highest rates of influenza-associated respiratory and circulatory hospitalizations over the past ten years. Children under the age of five had hospitalization rates similar to those of adults aged 50-64 years. Other groups at risk include pregnant women and persons with underlying health conditions.