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.
There are three types of flu viruses: A, B, and C. Influenza A and B cause the yearly epidemics of seasonal flu. Influenza C causes milder illness and is not believed to cause epidemics. Influenza A can infect a range of mammals and birds as well as humans, while types B and C infect mainly humans. Flu viruses are constantly changing via mutations, which is why you need a new flu vaccine every year.