Influenza, also known as the flu, is a highly contagious infection of the breathing system (nose, throat and lungs) caused by influenza viruses.
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Seasonal influenza is caused by viruses that already circulate
among people. Most people have some immunity and a vaccine is available
to help prevent the flu. In the United States, flu season usually
starts in late fall and lasts throughout the winter, sometimes into
early spring.
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Avian influenza, also called bird flu, is caused by viruses that
occur naturally among wild birds. Sometimes these viruses (such as one
called H5N1) can be transmitted from birds to humans. There is no human
immunity and no vaccine is available.
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Pandemic influenza refers to an outbreak caused by a new flu
virus that spreads around the world. The disease can spread easily from
person to person because people have little natural immunity. There is
no vaccine ready to protect against pandemic influenza because the new
virus strain must be identified before a vaccine can be produced, which
can take up to six months.
Currently, there is no human flu pandemic.