Every year the human population suffers from seasonal outbreaks of influenza resulting in both illness and death. However, the rates of illness and death from seasonal outbreaks are significantly lower than those suffered during times of influenza pandemic, such as those experienced in 1918, 1957, and 1968. The reason for this difference lies in presence of immunity within a population. With seasonal outbreaks of influenza most people have some immunity to the circulating strain and usually only those with weakened immune systems experience serious complications. Influenza pandemics, in contrast, are the result of a completely new viral subtype to which nobody possesses an immunity, leaving everyone vulnerable to the most serious of complications.