"I MUST KNOW THIS ABOUT INFLUENZA"
The History of the Flu Influenza A and B viruses continually undergo a type of change called antigenic drift. This process accounts for most of the changes that occur in the viruses from one influenza season to another. Another change -- called antigenic shift -- occurs only occasionally. When it does occur, large numbers of people, and sometimes the entire population, have no antibody protection against the virus. This may result in a worldwide epidemic, called a pandemic. During the last century, major pandemics occurred three times, each of which resulted in large numbers of deaths: 1918-19 "Spanish flu" A -- Caused the highest known influenza-related mortality: approximately 500,000 deaths occurred in the U.S., 20 million worldwide 1957-58 "Asian flu" A -- 70,000 deaths in the United States 1968-69 "Hong-Kong flu" A -- 34,000 deaths in the United State The Flu Viruses Flu viruses are divided into three...