What is the flu?

Let's pause here and take a moment to get a clearer picture of what the "flu" really is. The common day-to-day use of the term "flu" has evolved to often encompass anything from a "common cold" to a true "flu". The "stomach flu" is another misleading term, often used to describe a gastrointestinal illness (the "stomach flu" is usually not even caused by a virus but by other microorganisms). So, the first critical point to understand is that the "flu" is not a common cold or a stomach infection.

When researchers, or doctors speak of the "flu", they are being very specific and mean an infection by the influenza virus. Epidemic influenza is divided into type A and type B. The most common presentation of influenza includes a fever (usually 100-103 degrees F in adults), respiratory symptoms (such as cough, sore throat, runny or stuffy nose), headache, muscle aches, and, often extreme fatigue. So, the second key point is that public health officials and doctors mean influenza virus when they use the term "flu".

The year-in, year-out "flu" can be deadly (in an average year influenza is associated with about 20,000 deaths), especially for babies, the elderly, people with compromised immune systems, or those who have an existing chronic health condition, such as asthma, diabetes or heart disease. Even for those of us who are in generally good health, the "flu" can still really "take the wind out of our sails", causing us to feel miserable for several days to a week or two.

Currently there are several main subtypes of the "flu" in circulation. These subtypes are broken down into 2 families -- type 'A' and type 'B'. The type A family subtypes are the "Hong Kong" type A(H3N2) virus and its relatives (responsible for about 400,000 deaths in the United States since 1968 (90% of which are among the elderly), and distant relatives of the "Spanish Flu", type A(H1N1). Since 2001, a new subtype of influenza A - type A(H1N2) - has been found in several parts of the world including North America. Whether this new flu subtype will go into long-term widespread circulation and cause the same problems as the other type A subtypes is uncertain. Only one form of type B influenza virus is in circulation. When describing the subtype of influenza, the "H" and "N" refer to viral proteins called haemagglutinin (H) and the neuraminidase (N) (more on this in a bit).

Many medical experts consider the influenza virus (cause of the "flu") to be the most dangerous virus in the world. Each year about 20,000 people in the U.S. die from the flu. Several times in past history, this virus has been responsible for killing huge numbers of people within a 1 to 2 year period. As an example, the "Spanish flu" (type A(H1N1)) of 1918-19 killed about 500,000 people in the U.S. and at least 20 million people worldwide. In 1957-58, the "Asian flu" (type A(H2N2)) resulted in 70,000 deaths in the U.S., and in 1968-69, the "Hong-Kong flu" (type (A(H3N2)) killed 34,000 in the U.S. Some medical and public health experts believe it is only a matter of time (in fact they think we are overdue) before a new pandemic (worldwide epidemic) of the "flu" occurs, killing many, many people.