There are three forms of the influenza virus, A, B, and C. According
             to the Centers for Disease Control, the A and B forms of the virus infect
             millions of people each year and have been the source of flu epidemics.
             The influenza A form of the virus is the deadliest and is believed to be
             the source of the 1918, 1957 and 1968 flu epidemics. (Background: What We
             Know about the Flu) The CDC explains that influenza C is not thought to
             cause wide spread epidemics and that the flu shot does not protect against
             influenza C. (The Influenza (Flu) Viruses 2004)
             The Centers for Disease Control also reports that the Influenza A
             viruses are found in both animals and humans. The CDC explains that the A
             viruses are placed into subtypes based on proteins that are found on the
             surface of the virus. (The Influenza (Flu) Viruses 2004) The two proteins
             found on the surface of the virus are neuraminidase (NA) and hemagglutinin
             (HA). (The Influenza (Flu) Viruses 2004) The CDC goes on to explain that
             there are nine forms of the neuraminidase protein and fifteen forms of the
             hemagglutinin protein. (The Influenza (Flu) Viruses 2004)
             The CDC also reports that Influenza is spread amongst human beings
             through the sneezing and cough of an infected individual. Incubation time
             for influenza is one to four days. (Clinical Description and Diagnosis
             2003) The CDC explains that adults are infectious from the day before they
             experience symptoms until 5 days after the onset of the virus. (Clinical
             Description and Diagnosis 2003) On the other hand, children are usually
             infectious for 10 days and can be infectious for 6 days before the onset of
             the virus. (Clinical Description and Diagnosis 2003) The CDC also reports
             that individuals that are severely immunocompromised can be infectious for
             months. (Clinical Description and Diagnosis 2003)
             The symptoms of influenza include respiratory pr...