Patient Basics: Pyelonephritis (Kidney Infection)

Pyelonephritis is an infection of one or both kidneys. It usually starts as a bladder infection, when germs travel up the thin tubes (ureters) that connect the bladder to the kidneys. It causes fever, pain in the side or back and a sick, achy feeling. It needs antibiotics right away, because an untreated kidney infection can spread to the blood.

Key points

• Most kidney infections are caused by E. coli, a germ that normally lives in the bowel.
• Fever, chills and pain in the side or back are the signs that set it apart from a simple bladder infection.
• A urine test and a urine culture confirm the infection and show which antibiotic will work.
• Many people can take antibiotic pills at home. People who are very ill, vomiting or pregnant are usually treated in the hospital.
• You should start to feel better within 2 to 3 days. If you don’t, call your doctor.

What causes pyelonephritis

Germs from the skin around the genitals and anus can get into the urethra (the tube that carries urine out of the body). From there they can reach the bladder and climb up to a kidney. Less often, germs reach the kidney through the blood.

Things that raise the risk include:

• Being female. Women have a shorter urethra, so germs reach the bladder more easily. Sex can push germs toward the opening.
• Pregnancy. Hormones relax the ureters and the growing uterus presses on them, so urine drains more slowly.
• Anything that blocks urine. Kidney stones, an enlarged prostate or a narrowed ureter let urine sit and germs grow.
• A urinary catheter or a recent procedure on the urinary tract.
• Diabetes or a weak immune system.
• Urine reflux in children. In vesicoureteral reflux, urine flows backward from the bladder toward the kidneys.

Symptoms

Symptoms often come on over a day or two:

• Fever, often above 100.4°F (38°C), and shaking chills
• Pain in the side, back or groin, usually on one side just below the ribs
• Nausea and vomiting
• Burning when you pass urine, needing to go often or urgently
• Cloudy, bloody or bad-smelling urine
• Feeling very tired and unwell

Older adults may have few of these signs. Confusion, a fall or sudden weakness may be the main clue. Babies and young children may only have a fever, be fussy, feed poorly or vomit.

How pyelonephritis is diagnosed

• Exam. Your doctor checks your temperature, heart rate and blood pressure, and taps gently over your lower back to see if a kidney is tender.
• Urine test. A clean sample is checked for white blood cells, blood and germs.
• Urine culture. The lab grows the germ to find out which antibiotics will kill it. This takes a day or two, so treatment usually starts before the result is back.
• Blood tests check for signs of a serious infection and see how well your kidneys are working. If you are very ill, blood cultures look for germs in the blood.
• A pregnancy test for women who could be pregnant.
• Imaging. A CT scan or ultrasound is done if you are very sick, aren’t getting better after 2 to 3 days, or might have a stone or blockage. Children with a kidney infection often have an ultrasound to check the kidneys’ shape.

Pyelonephritis treatment

At home

If you can drink fluids and keep pills down, you can usually be treated at home. Your doctor may give a first dose of antibiotic as a shot or through a vein, then start pills. Common choices include ciprofloxacin, levofloxacin, trimethoprim-sulfamethoxazole or certain cephalosporins, depending on what germs are common where you live and on your culture results. Treatment lasts about 5 to 14 days, depending on the drug. Some antibiotics used for bladder infections, such as nitrofurantoin and fosfomycin, don’t reach the kidney well and aren’t used for this infection.

Take every dose, even after you feel better. Drink plenty of fluids and rest. A heating pad on your back or side can ease pain. Acetaminophen helps with fever and aches. Ask before taking ibuprofen or naproxen, which can strain the kidneys when you are dehydrated.

In the hospital

Hospital care is needed if you are vomiting, very ill, pregnant, have signs of a blood infection, or have a blockage. You get antibiotics and fluids through a vein. If a stone or something else is blocking the kidney, a doctor drains it with a small tube placed through the bladder (a stent) or through the skin of the back. A pocket of pus in the kidney (an abscess) may also need draining. Germs that resist the usual pills often need IV antibiotics. A newer antibiotic pill, tebipenem, has been tested for these hard-to-treat infections.

Recovery

Fever and pain usually ease within 48 to 72 hours of starting the right antibiotic, though tiredness can last a week or more. Most healthy adults recover fully. Repeated infections, especially in young children, can scar the kidneys over time. In pregnancy, kidney infections can lead to early labor, so they are treated quickly and watched closely.

Lowering your risk

• Get a bladder infection treated early, before it can spread.
• Drink enough fluid that your urine stays pale.
• Go when you feel the urge, and pass urine soon after sex.
• Wipe from front to back after using the toilet.
• If you’ve gone through menopause and get repeat infections, ask about vaginal estrogen, which lowers the risk.
• Cranberry products may cut repeat bladder infections in some women. They don’t treat an active infection.

Pregnant women are tested for germs in the urine early in pregnancy, even without symptoms, because treating them lowers the chance of a kidney infection.

When to see a doctor

• Fever, chills, or side or back pain with or without bladder symptoms
• Bladder infection symptoms during pregnancy
• No improvement after 2 to 3 days of antibiotics
• Symptoms that come back after treatment
• A fever in a baby or young child with no clear cause

Get emergency care for confusion, fainting, a racing heart, fast breathing, cold or blotchy skin, or vomiting so much that you can’t keep fluids or pills down. These can be signs of sepsis, a life-threatening reaction to infection.

Common questions about pyelonephritis

Is a kidney infection serious?

It can be. Most kidney infections clear completely with the right antibiotic, but without treatment the germs can spread into the blood and cause sepsis, which can be life-threatening. Repeated infections can also scar the kidneys. That’s why fever and back pain along with urinary symptoms need a doctor’s visit the same day.

Is pyelonephritis contagious?

No. You can’t catch a kidney infection from another person the way you catch a cold. The germs usually come from your own bowel and travel up the urinary tract. Sex can move germs toward the urethra, though, which is one reason to pass urine soon afterward.

How can I tell a kidney infection from a bladder infection?

A bladder infection causes burning, frequent urination and pressure low in the belly, usually without a fever. A kidney infection adds fever, chills, pain in the side or back, and often nausea or vomiting. If you have bladder symptoms plus any of these, see a doctor promptly.

Questions to ask your doctor

• Do I need to be in the hospital, or can I recover at home?
• How long should I take the antibiotic?
• Do I need a scan to look for a stone or blockage?
• What should I do if my fever hasn’t gone down in two days?
• How can I prevent another infection?

Learn more

• Kidney infection (pyelonephritis) (National Institute of Diabetes and Digestive and Kidney Diseases)

This guide is general information and doesn’t replace advice from your own doctor. Last updated October 2026.