• The influenza virus changes, or mutates, every year.
• These changes cause seasonal flu epidemics.
• These changes also necessitate the reformulation of the flu vaccine every year.

• Influenza is spread via respiratory secretions from infected persons.
• The incubation period is from 1 to 4 days.
• Attack rates range from 15-40% in a normal year.
• Infected persons "shed" virus for about a week, although children and immunocompromised patients shed virus for a longer period of time.

Influenza is caused by one of two types of influenza viruses - type A and type B. The type A virus is further classified into subtypes. This classification is based on two surface antigens, hemagglutinin (H) and neuraminidase (N).

The influenza virus changes each year - sometimes a little, sometimes a lot. Major changes, known as subtype changes or antigenic shifts, produce worldwide epidemics called pandemics. In 1918, the influenza A virus subtype H1N1, known as the "Spanish Flu," killed 500,000 people in the U.S. and 20 million people globally. In 1957, the "Asian Flu" (H2N2) killed 70,000 in the U.S., and in 1968 the "Hong Kong Flu" (H3N2) caused another epidemic.

Minor viral changes are called strain changes (or antigenic drift), but they are not always so minor. In the 1997-1998 flu season, the "A/Sidney" strain caused almost double the U.S. yearly average of 20,000 influenza-related deaths. These yearly changes demand yearly vaccine alterations.

The next pandemic can occur at anytime. It will be a different virus subtype, one that may never have been encountered by humans. Because we will have no immunity, it will kill many more than the average of 20,000 people in the U.S. Even young healthy adults will have a severe illness. The National Pandemic Preparedness Task Force is preparing for this scary event to come.

Influenza virus in humans is spread via respiratory secretions from infected persons, through coughing and sneezing. These aerosols of viruses contaminate the air, hands, and surfaces of our environment. The incubation period is very short (from 1 to 4 days), and attack rates range from 15-40% in a normal year. In nursing homes, 60 out of 100 patients can be affected, with up to 30 fatalities. A person over age 65 is at risk because the immune system is weakened and, over the years, "forgets" it has been exposed to influenza.

Sick persons "shed" the virus and are contagious for about a week. Immunocompromised persons (e.g., those with cancer, AIDS, or who may be on immunosuppressants) easily contract influenza and shed virus for 2-3 weeks.

School-age children are the first to become ill in each community and are the primary source of contagion since they shed virus for 7 to 10 days. Because they have little or no previous exposure to influenza, children have less immunity and can experience more severe forms of flu.

Others at risk for serious influenza complications are patients with asthma, pulmonary disease, heart disease, diabetes, renal disease and anemia.