Influenza

INFLUENZA

Influenza, commonly called "the flu," is an illness caused by viruses that infect the respiratory tract. Compared with most other viral respiratory infections, such as the common cold, influenza (flu) infection often causes a more severe illness with a mortality rate (death rate) of about 0.1% of people who are infected with the virus. Unusually severe worldwide outbreaks (pandemics) have occurred several times in the last 100 years since influenza virus was identified in 1933. By an examination of preserved tissue, the worst influenza pandemic occurred in 1918 when the virus caused between 40 to 100 million deaths with a mortality rate estimated to range from 2% to 20%.

Influenza 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 of 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 of influenza types A and B. Influenza type A viruses undergo two kinds of changes. One is a series of mutations that occurs over time and causes a gradual evolution of the virus. This is called antigenic "drift." The other kind of change is an abrupt change in the proteins. This is called antigenic "shift." In this case, a new subtype of the virus suddenly emerges. Type A viruses undergo both kinds of changes; influenza type B viruses change only by the more gradual process of antigenic drift and therefore do not cause pandemics.

Typical flu symptoms include: fever, respiratory symptoms such as cough, sore throat, and a runny or stuffy nose, headache, muscle aches, and fatigue. Vomiting, nausea, and diarrhea can also accompany influenza infection. Most people who get the flu recover completely in one to two weeks, but some people develop serious and potentially life-threatening medical complications, such as pneumonia. In an average year, influenza is associated with about 20,000 deaths nationwide and many more hospitalizations. Flu-related complications can occur at any age; however, the elderly and people with chronic health problems are much more likely to develop serious complications after influenza infection than are younger, healthier people.

Much of the illness and death caused by influenza can be prevented by annual influenza vaccination. Flu vaccine is specifically recommended for those who are at high risk for developing serious complications as a result of influenza infection. These high-risk groups include all people