Flu is the common name for a variety of similar highly infectious viral diseases which tend to cause fever, aches, lethargy and related symptoms in infected people. Flu tends to appear seasonly, in the late fall and winter, for reasons that aren't terribly well understood. There are multiple strains of the flu and unlike many diseases against which people are commonly vaccinated they vary considerably from year to year. Generally speaking, because flu is caused by a virus, rather than a bacteria, medical science can only treat the symptoms once they appear, rather than curing the cause of the flu. Modern science has many effective antibiotics, but few effective antiviral drugs (i.e. drugs that effectively suppress viruses once they infect the body). As a result, preventing flu with a vaccine is the only effective medical method to address the cause of this disease. A vaccine is a way of training a person's own immune system to more effective fight a disease in the future.
Flu vaccination is generally done in the fall each year as a Public Health measure. Because strains vary (and possibly because the immunity conferred by the vaccine may not be as long lasting as some other kinds of vaccines), maximum immune resistance to the flu requires an annual vaccine shot. The strains that the flu vaccine protects against need to be changed each year, and if vaccine developers guess wrong, even widespread vaccination will be ineffective. But, if the vaccine developers guess right (and usually they do), many deaths can be prevents by vaccinating high risk individuals and people likely to come into contact with them.
In healthy adults and for older healthy children, flu in an unpleasant inconvenience, but for young children, the elderly and people with compromised immune systems, flu can be deadly. For example, in the year 2000, 67,044 people died of pneumonia and influenza (mostly those in the vulnerable groups) making it the 7th leading cause of death in the United States (although the death rate is still 88% lower than in 1900 and about 20% lower than it was in 1970). Put another way, if flu vaccination produces even a conservative 10% reduction in deaths, this is equivalent to a 40% reduction in the homicide rate in terms of lives saved, at cost on the order of 0.1% of the money the country spends on Medicare (see Health Care) each year.
Flu vaccines are primarily produced by just a few drug companies. In 2004 there were only three major producers of flu vaccines and one of them, a British company that produces flu vaccines, had its drug company license suspended before it could produce its expected quota of vaccines. The 2004 flu vaccine shortage has raised numerous public policy issues regarding how the Bush Administration has handled this Public Health issue and with how it has addressed the economics of Health Care more generally.