Influenza a.k.a The Flu
Influenza, commonly referred to as the flu, is a virus that can affect warm blooded animals. In minor cases it can cause symptoms such as a headache, fever, soar throat, muscle weakness, and sore joints. Many times it will go away in a few days. However, in some cases it can progress to upper respiratory infections, such as pneumonia, which can be fatal. In fact, what most people don't realize, is that every year thousands of people die from influenza. Even developed countries are not spared the ravages of the virus.
Because of its severity many governments promote the flu vaccine, and distribute it in the fall, before 'flu season.' The difficulty is that the flu, being a virus, can mutate into different types. So, the developed vaccine can be quite ineffective between the time the vaccine is developed and the time flu season actually arrives. However, most experts believe that those susceptible should still obtain the vaccine. The mutation of the virus is also why every year a new vaccine has to be developed.
Generally speaking, the elderly, and those suffering from a weak immune system, are most susceptible to influenza.
Vaccines contain inactive proteins from the influenza strain. By administering this into the body the immune system develops anti-bodies, which will destroy the 'real' influenza during a real infection. Basically, it prepares the body's immune system ahead of time to do battle.
It is important to note that the common cold and influenza are totally separate. The common cold is not nearly as dangerous as influenza. In fact, people usually recover from the common cold within a few days. The reason for the confusion is that both occur more often at the same time of the year, and both have similar symptoms. There is no vaccine for the common cold, however, once the body has been exposed to a specific strain their body is able to fight future exposures much more efficiently. And, despite all the marketing ventures, there is no known cure for the common cold. Most medicines simply mask some of the symptoms and do very little in eliminating the virus.