Flu or Influenza is a viral infection that attacks the respiratory system, including your nose, throat, bronchial tubes and lungs. Although it's commonly called the flu, influenza is not the same as the stomach virus that causes 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. Although the shot doesn't offer 100 percent protection, it can reduce your chance of infection and help prevent serious complications if you do get sick.