Definition & Facts of Kidney Infection (Pyelonephritis)

In this section:

What is a kidney infection?

A kidney infection is a type of urinary tract infection (UTI). Most kidney infections are caused by bacteria or viruses that first infect your lower urinary tract, usually your bladder. Then, the infection moves upstream to one or both of your kidneys, which are part of the upper urinary tract.

In some cases, you can get a kidney infection after surgery if bacteria enter your body during the procedure and travel through your blood to the kidneys.

Your body has ways to defend against infections in the urinary tract. For example, urine normally flows one way from your kidneys to your bladder. Viruses or bacteria that enter are flushed out by urinating. This one-way flow of urine usually prevents an infection in your urinary tract. Learn more about your urinary tract and how it works.

Sometimes your body’s defenses fail and bacteria or viruses cause a UTI in the bladder. If you have symptoms of a bladder infection, see a health care professional. You may need treatment to prevent the infection from spreading to your kidneys. Kidney infections are often very painful and can cause serious health problems.

Urinary tract which includes the kidneys, ureters, bladder, and urethra.
Kidney infections often begin in the bladder, and then move up through the ureters to infect one or both kidneys.

Does kidney infection have another name?

Yes, the medical term for kidney infection is pyelonephritis.

How common are kidney infections?

Kidney infections cause most of the 100,000 hospital visits for UTIs in the United States each year.1

Who is more likely to develop a kidney infection?

You are more likely to develop a kidney infection if you

  • Are a woman2
  • have a UTI in the bladder.
  • had a UTI during the past 12 months.
  • are pregnant. Scientists think that hormonal changes and shifts in the position of the urinary tract during pregnancy make it easier for bacteria to travel to the kidneys and cause infection.
  • have a problem in your urinary tract that blocks or changes the normal flow of urine. The flow of urine may be blocked if you have a defect in the structure of your urinary tract, such as a narrowed urethra, an enlarged prostate, or a kidney stone.
  • have vesicoureteral reflux (VUR), which is when urine can back up, or reflux, into one or both kidneys. Health care professionals most commonly diagnose VUR in children.3
  • have diabetes or problems with your body’s immune, or natural defense, system.
  • have a spinal cord injury or nerve damage around the bladder.
  • have trouble emptying your bladder completely, called urinary retention.
A pregnant woman and her male partner walking on a beach.
Kidney infections are more common in women than men and also are more common in pregnant women.

What are the complications of kidney infections?

In rare cases, kidney infections may cause

Your chance of a complication is slightly greater if you have

  • kidney disease from other causes
  • a problem with the structure of your urinary tract
  • repeated episodes of kidney infection

Complications from a kidney infection are rare if a health care professional prescribes antibiotics to treat your infection.


Last Reviewed April 2017
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This content is provided as a service of the National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK), part of the National Institutes of Health. NIDDK translates and disseminates research findings to increase knowledge and understanding about health and disease among patients, health professionals, and the public. Content produced by NIDDK is carefully reviewed by NIDDK scientists and other experts.

The NIDDK would like to thank:
Ann E. Stapleton, MD, FIDSA, FACP, University of Washington School of Medicine