Anyone who thinks that doesn't know much about history of flu pandemics -- that is, outbreaks of disease that spread around the globe. The dreaded Spanish flu, a particularly vicious strain that emerged in 1918, killed between 20 and 50 million people worldwide, more than the number who died in World War I. Another strain, the Hong Kong flu, killed about 1 million people in 1968-69 [source: Sample].
While the flu's symptoms -- fever, coughing, a painfully sore throat, muscular aches, headaches and extreme fatigue -- are plenty debilitating, most people recover in anywhere from a few days to two weeks [source: CDC]. In severe cases, a flu victim can also get severe dehydration and sepsis -- toxins in the blood that can cause the body's organs to go haywire and begin to fail [source: Sepsis Alliance].
But the real risk may not be from the flu virus's direct effects, but how it weakens the body and makes a person vulnerable to other illnesses, some of which can be fatal. Many seasonal flu-related deaths actually occur a week or two after a person comes down with the flu, when they develop a secondary bacterial infection, such as bacterial pneumonia. In other cases, the flu aggravates an existing chronic illness, such as congestive heart failure or chronic pulmonary heart disease [source: CDC]. People with asthma may experience potentially fatal attacks of that ailment when they come down with the flu [CDC].