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Influenza is a very contagious viral disease. Almost everyone exposed to the disease who has not been recently vaccinated will develop symptoms, however in most patients they are mild and the patient recovers with simple care like replacing fluids. However, influenza is often fatal to the elderly or those with pre-existing respiratory conditions, such as asthma.

Influenza mutates very quickly and, as such, having the disease is no guarantee that the patient will not contract another strain of the disease at a later date. The flu shot is tailored every year for the most common strains in circulation at the time.

Despite being easy to prevent and its lack of severe symptoms in most patients, influenza is one of the contagious diseases that kills the most people every year. By way of comparison, during Toronto's SARS crisis, more people actually died of influenza than SARS in Toronto during the period where the mortality rate from SARS was the highest.

Influenza at Wikipedia

Influenza at Mayo Clinic