Influenza is a viral infection that attacks the respiratory system, including your nose, throat, bronchial tubes and lungs. Influenza, commonly called the flu, is not the same as the stomach viruses that cause diarrhea and vomiting.

Anyone can get the flu, but young children, older adults, people with weakened immune systems and those with chronic illnesses are especially vulnerable. If you're at high risk of flu, your first line of defense is an annual flu shot.

Approximately 50 million people died worldwide in the 1918 influenza pandemic. Today, according to the Centers for Disease Control and Prevention, as many as 36,000 people in the United States die each year of complications of influenza and more than 200,000 are hospitalized.