Updated January 21, 2012.

Definition:

Tonsillitis is an infection of the tonsils, glands that rest in the back of the mouth. Tonsils are part of the infection-fighting immune system and can become infected themselves.

When tonsils become infected, they become red, swollen and very painful. If they become infected repeatedly, a surgery to remove the tonsils, a tonsillectomy, may be necessary.

Common Misspellings: tonsilitis,

Examples: After looking in the girl's mouth, the doctor determined that she had tonsillitis.