Signs and symptoms of a kidney infection might include:

• Fever
• Chills
• Back, side (flank) or groin pain
• Abdominal pain
• Frequent urination
• Strong, persistent urge to urinate
• Burning sensation or pain when urinating
• Nausea and vomiting
• Pus or blood in your urine (hematuria)
• Urine that smells bad or is cloudy

Bacteria that enter your urinary tract through the tube that carries urine from your body (urethra) can multiply and travel to your kidneys. This is the most common cause of kidney infections.

Bacteria from an infection elsewhere in your body also can spread through your bloodstream to your kidneys. Although it's unusual to develop a kidney infection, it can happen — for instance, if you have an artificial joint or heart valve that becomes infected.

Rarely, kidney infection results after kidney surgery.

Factors that increase your risk of a kidney infection include:

• Being female. The urethra is shorter in women than it is in men, which makes it easier for bacteria to travel from outside the body to the bladder. The nearness of the urethra to the vagina and anus also creates more opportunities for bacteria to enter the bladder.

Once in the bladder, an infection can spread to the kidneys. Pregnant women are at even higher risk of a kidney infection.

• Having a urinary tract blockage. This includes anything that slows the flow of urine or reduces your ability to empty your bladder when urinating — including a kidney stone, something abnormal in your urinary tract's structure or, in men, an enlarged prostate gland.
• Having a weakened immune system. This includes medical conditions that impair your immune system, such as diabetes and HIV. Certain medications, such as drugs taken to prevent rejection of transplanted organs, have a similar effect.
• Having damage to nerves around the bladder. Nerve or spinal cord damage can block the sensations of a bladder infection so that you're unaware when it's advancing to a kidney infection.
• Using a urinary catheter for a time. Urinary catheters are tubes used to drain urine from the bladder. You might have a catheter placed during and after some surgical procedures and diagnostic tests. You might use one continuously if you're confined to a bed.
• Having a condition that causes urine to flow the wrong way. In vesicoureteral reflux, small amounts of urine flow from your bladder back up into your ureters and kidneys. People with this condition are at higher risk of kidney infection during childhood and adulthood.

If left untreated, a kidney infection can lead to potentially serious complications, such as:

• Kidney scarring. This can lead to chronic kidney disease, high blood pressure and kidney failure.
• Blood poisoning (septicemia). Your kidneys filter waste from your blood and return your filtered blood to the rest of your body. Having a kidney infection can cause the bacteria to spread through your bloodstream.
• Pregnancy complications. Women who develop a kidney infection during pregnancy may have an increased risk of delivering low birth weight babies.