"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 types, designated A, B, and C. Influenza types A and B are responsible for epidemics of respiratory illness that occur almost every winter and are often associated with increased rates for hospitalization and death.

Influenza type C differs from types A and B in some important ways. Type C infection usually causes either a very mild respiratory illness or no symptoms at all; it does not cause epidemics and does not have the severe public health impact that influenza types A and B do. Efforts to control the impact of the flu are aimed at types A and B, and the remainder of this discussion will be devoted only to these two types.

Flu viruses continually change over time. 

This constant changing enables the virus to evade the immune system, so that people are susceptible to the flu throughout life. 

This process works as follows: a person infected with a flu virus develops antibodies against that virus; as the virus changes, the "older" antibodies no longer recognizes the "newer" virus, and the person gets sick. 

The older antibodies can, however, provide partial protection against newer viruses.