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Wintertime is influenza season in the Northern Hemisphere. Influenza is frequently referred to as "the flu." This is somewhat confusing because the term "flu" is also used as a nonspecific diagnostic term, much like the term "cold," in a variety of viral infections ("stomach flu" or "respiratory flu"). Actually, influenza is a very specific, virulent virus, occurring in yearly epidemics which appear to be increasing in severity.

Its symptoms include high fevers lasting several days, watery eyes, runny nose, deep cough, sore throat, vomiting, and headaches frequently lasting a week or two. It is not only a cause of long-lasting misery; it is a leading cause of hospitalizations and death worldwide.

Although influenza mostly causes respiratory symptoms, including pneumonia and croup, complications include ear infections, convulsions, muscle damage, brain inflammation, and heart damage. In the U.S., about 30,000 deaths per year are attributed to influenza and its complications. The Center for Disease Control and Prevention (CDC) estimates that the numbers will more than double in the next 20 years unless we do a better job of infection control.

The most vulnerable population to influenza illness is children under 5 years of age, and the most vulnerable populations to death from influenza complications are the elderly and young children. An additional vulnerable group includes anyone with chronic illness, such as asthma and heart disease, and people with immune difficulties.

Time for some scary information. It appears that the influenza pandemic (extensive worldwide epidemic) of 1918-1920 was the worst to ever strike the U.S. It resulted in 675,000 deaths (about 1% of the U.S. population), mostly in young adults, and despite recovering the virus from victims of that epidemic, we still cannot identify what factors made that influenza virus so potent.

More scary facts. The origin and main reservoir of influenza virus is probably bird species. The influenza virus has 15 known hemaglutinin surface antigens (the main factors determining its attack mechanism). These antigens begin their attack by attaching to the respiratory lining in birds. So far, only 3 have learned to attach to the respiratory lining of humans. If and when the others learn how to do this then we are in for quite a bit of havoc. It is theorized that this occurs when one of these antigens first adapts to a mammal such as swine. Further adaptation or mutation of this antigen occurs and humans become a new target. Every time that this happens a new antigen is introduced.

This has the potential for causing a severe epidemic until a vaccine or effective therapy becomes available. Since only 3 of the antigens have learned to attack humans, there are still 12 potential additional attackers. This is why all of the poultry was destroyed in Hong Kong in 1997 when infected poultry flocks were discovered. This information should be great fodder for all of you budding science-fiction writers.

Time for some good news. We have the tools to do a much better job in controlling influenza. Besides public health measures such as surveillance, advocating appropriate hygiene (thorough hand-washing and discouraging sharing food and towels), and quarantine (keeping sick children and adults away from others), vaccines and anti-influenza medications are available.

(Next issue more on influenza--vaccines and treatment.)

Dr. Pellman's column has appeared monthly in Pediatrics for Parents for over 10 years. He's a Board Certified Pediatrician practicing in Huntington Beach, California, a Clinical Professor of Pediatrics at the University of California.