Influenza, commonly called "the flu," is caused by viruses that infect the respiratory tract. Compared with most other respiratory infections, such as the common cold, the flu often causes a more severe illness.
Typical flu symptoms include fever (usually 100-103 degrees Fahrenheit 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 often extreme fatigue. Although nausea, vomiting, and diarrhea can sometimes accompany the flu, especially in children, gastrointestinal symptoms are rare. The term "stomach flu" isn't really a flu at all. It's often used to describe an illness caused by other viruses.
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Most people who get the flu recover completely in one to two weeks, but some people develop serious and potentially life-threatening medical complications, such as pneumonia. Because each flu season is different in length and severity, the number of serious illnesses and deaths that occur each year varies. In the past 30 years, the annual death rate from flu-related causes has ranged from 3,000 to 49,000 deaths per year. Flu-related complications can occur at any age; however, very young children, pregnant women, the elderly, and people with chronic health problems are much more likely to develop serious complications of the flu than are younger, healthier people.
The Flu Viruses
Flu 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 the flu are aimed at types A and B, and the remainder of this discussion will be devoted only to these two types.
Flu viruses continually change over time. This constant changing enables the virus to evade the immune system, so that people are susceptible to the flu throughout life. This process works as follows: a person infected with a flu virus develops antibodies against that virus; as the virus changes, the "older" antibodies no longer recognizes the "newer" virus, and the person gets sick. The older antibodies can, however, provide partial protection against newer viruses.
The History of the Flu
Influenza A and B viruses continually undergo a type of change called antigenic drift. This process accounts for most of the changes that occur in the viruses from one influenza season to another.
Another change -- called antigenic shift -- occurs only occasionally. When it does occur, large numbers of people, and sometimes the entire population, have no antibody protection against the virus. This may result in a worldwide epidemic, called a pandemic. During the last century, major pandemics occurred three times, each of which resulted in large numbers of deaths:
1918-19 "Spanish flu" A -- Caused the highest known influenza-related mortality: approximately 500,000 deaths occurred in the U.S., 20 million worldwide
1957-58 "Asian flu" A -- 70,000 deaths in the United States
1968-69 "Hong-Kong flu" A -- 34,000 deaths in the United States
CDC: "Estimating Seasonal Influenza-Associated Deaths in the United States: CDC Study Confirms Variability of Flu."
CDC: "Key Facts about Influenza (Flu) & Flu Vaccine."
CDC: "Types of Influenza Viruses."
National Institute of Allergy and Infectious Diseases: "Timeline of Human Flu Pandemics."
MedlinePlus: "Common Cold."