The standard deviation is a measure of how the data is clustered about the mean. For large sets of data, approximately 68.3% of the data lies within one standard deviation of the mean and approximately 95.4% of the data lies within two standard deviations of the mean.
Influenza (also known as the flu) is a contagious respiratory illness caused by influenza viruses. It can cause mild to severe illness, and at times can lead to death. The best way to prevent the flu is by getting a flu vaccination each year.
Other important facts. 1  Both colds and flu are caused by a virus but the symptoms of influenza are usually more severe. 2  Antibiotics have no effect against influenza. 3  Damp or cold cannot cause influenza. 4  People are seldom infected by the same type of influenza twice because resistance develops within two or four weeks.