What is flu?
The flu (or common flu) is a viral infection that is spread from person to person in secretions of the nose and lungs, for example when sneezing. Medically, it is referred to as influenza. Flu is a respiratory infection, that is, an infection that develops primarily in the lungs. Respiratory infections caused by other viruses often are called flu, but this is incorrect. Influenza usually causes higher fever, more malaise, and severe body aches than other respiratory infections. Although other viruses may cause these symptoms, they do so less commonly.
Influenza viruses are divided scientifically 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. Influenza type C 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 of influenza types A and B. Type A viruses are divided into subtypes and are named based on differences in two viral surface proteins called hemagglutinin (H) and neuraminidase (N). There are 16 known H subtypes and nine known N subtypes.
The flu is a common illness. Every year in the United States, on average
5%-20% of the population gets the flu,
more than 200,000 people are hospitalized from flu complications,
about 36,000 people die from the flu or its complications.
The “swine flu” pandemic of 2009 is caused by a novel influenza A virus designated H1N1 based upon its surface protein types. This virus was originally referred to as swine flu because many of the genes in this new virus were very similar to influenza viruses that normally occur in pigs in North America. However, this new virus is actually quite different from the typical swine flu viruses found in pigs. The novel H1N1 virus first caused illness in Mexico and the United States in March and April 2009. H1N1 flu is spread from person to person, unlike typical swine flu as described above, although it is not clear how easily the virus is able to spread among people.