A kidney infection occurs when bacteria travel from the bladder up the ureters and into the kidneys. Occasionally, kidney infections may occur when bacteria in the blood enter the kidneys.

In addition to painful urination and other symptoms of a bladder infection, symptoms of kidney infection also include:

• Pain on one side of the back, just below the rib cage and above the waist. This is often called flank pain.
• Fever.
• Nausea and vomiting.

If not treated with antibiotics, the infection may damage the kidneys. The infection may also spread to the bloodstream and become life-threatening, especially in young children and the elderly.