Understanding the flu has gotten more complicated, like it seems all things have. Influenza, or flu, is a respiratory infection caused by a variety of flu viruses. Flu viruses are classified as Type A, B, or C. Type A flu viruses also have a variety of subtypes. Type A viruses are the most common and cause the most serious epidemics. Type B can also cause epidemics but usually produce a much milder illness that type A. Type C have never been associated with epidemics. Seasonal flu is the term used to describe the flu outbreaks that occur annually, usually in the late fall and winter. About 5 to 20% of Americans come down with the flu each year. Pandemic flu is used to describe virulent strains of the flu that spread rapidly and cause a world-wide epidemic known as a pandemic. Bird or avian flu describes virulent bird influenza found in wild aquatic birds such as ducks and shore-birds that can infect humans and has caused a number of deaths in Asia. Normally bird flu is not spread directly to humans from birds. Pigs, however, are susceptible to both bird flu and human flu and can spread bird flu viruses to humans. In 1997 it was discovered that a virulent strain of bird flu skipped the pig stage to infect humans directly. This raised concerns that if this new strain of flu viruses began to be passed from human to human, it could possibly cause a pandemic because humans would have no immunity to this strain.

Flu often starts with symptoms similar to the common cold but is accompanied with high fevers, headaches, and extreme exhaustion rarely seen with colds. The flu can literally "knock you off your feet."

The typical flu season lasts from November to March. Children are 2 to 3 times more likely to get the flu than adults. Although most people recover from the flu without complication, it is estimated that over 200,000 people are hospitalized and about 36,000 die from complications of the flu each year. Flu outbreaks begin suddenly and spread rapidly throughout a community. During an outbreak, cases peak after about 3 weeks and subside after another 3 or 4 weeks. Half of a community population may be affected during an outbreak. Schools are an excellent place for flu viruses to spread, and as a result families with school-age children get more infections. Most healthy people who get the flu get better after about a week but the elderly, newborns, and people with certain chronic illnesses are at greater risk for severe cases of the flu that may be life-threatening.

Flu viruses contain 8 segments of single-strand RNA encoded with the genetic instructions for making new copies of the virus. The most striking features of these viruses are layers of spikes projecting from the surface. There are 2 types of surface spikes, the protein hemaggluttinin (HA) that allows the virus to stick to a cell and initiate infection, and the protein neuraminidase (NA) which allows newly formed viruses to exit the host cells. These proteins are used to determine which type of flu vaccine to produce. Unfortunately, flu viruses are among the most changeable. Small, continuous changes occur in type A and type B viruses as the viruses make copies of themselves, called antigenic drift. These changes are enough to make the virus unrecognizable to the human immune system, requiring that a new flu vaccine must be produced each year to prevent the most prevalent strains. Yearly vaccination is the best way to prevent getting the illness.

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Reference: http://www3.niaid.nih.gov/healthscience/healthtopics/Flu/understandingFlu/Default.htm.

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