Influenza is an infectious disease of the mouth, nose and upper airways caused by a flu virus.

In Europe, during the 15th and 16th centuries, people believed flu epidemics were influenced by the alignment and movement of the stars in the sky.

Symptoms of influenza infection include chills, cough, fever, headache, nasal congestion, sore throat, muscle and bone aches throughout the body. Infection usually has more widespread systemic symptoms through the body and tends to cause more severe symptoms than those of the common cold. Bronchitis and pneumonia can occur with an influenza infection and also with a secondary bacterial infection.

About 15%, one in seven, of people people in North America will get the flu during the year and 0.3% to 0.5% of those affected will develop a bad or serious case of the flu. A smaller percentage of those individuals will die from the infection. Older people, over the age of 65, with comorbidities such as cancer, diabetes, lung disease and who are overweight are at higher risk of more serious infections. Also, young children, under the age of five, are at higher risk.

The are four main types of influenza virus—A, B, C and D. There are also different subtypes within each group based on the proteins of the viral........

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