A kidney infection is a bacterial infection that develops in one or both kidneys, usually after bacteria travel upward from the bladder through the tubes (ureters) that connect the bladder to the kidneys. It’s a more serious form of urinary tract infection that can cause fever, back pain, and nausea, and it typically requires prompt antibiotic treatment. The overall incidence is about 4.2 cases per 10,000 people per year, with women affected more than four times as often as men.
How a Kidney Infection Develops
Most kidney infections start as a lower urinary tract infection, commonly called a bladder infection or cystitis. Bacteria, most often E. coli (responsible for 75 to 95 percent of cases), enter the urethra and multiply in the bladder. These bacteria have specialized surface proteins that let them cling to the walls of the urinary tract, resisting the natural flushing action of urine. If the infection isn’t cleared, the bacteria can climb from the bladder up through the ureters and invade the kidney tissue itself.
Less commonly, other types of bacteria cause kidney infections. Klebsiella and Proteus species account for a smaller share of cases. Repeated kidney infections caused by Proteus bacteria can be a sign of a specific type of kidney stone called a struvite stone, which forms in the presence of certain bacteria and can block urine flow.
Symptoms That Set It Apart From a Bladder Infection
A kidney infection shares some symptoms with a standard bladder infection, like burning during urination, frequent urges to pee, and cloudy or foul-smelling urine. What distinguishes it is how quickly and intensely you feel sick. A kidney infection is more likely to come on suddenly and make you feel systemically unwell, not just uncomfortable when you urinate.
The hallmark symptoms include:
- Fever and chills, which are rare with a simple bladder infection
- Pain in your lower back or side, typically on the side of the affected kidney
- Nausea or vomiting
- Bloody or cloudy urine
- Painful, frequent, or urgent urination
The back or side pain, often called flank pain, is one of the clearest signals. It tends to feel deep and aching, and it may hurt more when someone presses on the area or when you move. If you have symptoms of a bladder infection plus fever and flank pain, a kidney infection is the likely explanation.
Who Is Most at Risk
Women are far more susceptible to kidney infections because of anatomy. The female urethra is shorter, which gives bacteria a shorter path from outside the body to the bladder. The urethra’s proximity to the vagina and anus also increases the chance of bacterial contamination.
Beyond sex, several factors raise your risk:
- Anything that blocks urine flow. Kidney stones, an enlarged prostate, or a narrowed urethra can prevent the bladder from emptying fully, giving bacteria more time to multiply and ascend.
- Vesicoureteral reflux. This is a condition where small amounts of urine flow backward from the bladder toward the kidneys. People with this condition face a higher risk of kidney infections in both childhood and adulthood.
- A weakened immune system. Conditions like diabetes and HIV, or medications taken after an organ transplant, can reduce your body’s ability to fight off infections before they spread.
- Nerve damage around the bladder. If the nerves that signal when your bladder is full aren’t working properly, urine can sit in the bladder longer than it should.
- Urinary catheters or stents. Any device placed in the urinary tract can introduce bacteria or become a surface for bacterial growth.
- Pregnancy. Hormonal changes and the physical pressure of a growing uterus can slow urine flow and make infections more likely to reach the kidneys.
How It Is Diagnosed
Diagnosis usually starts with your symptoms and a physical exam. Your doctor will ask about the location and severity of your pain, whether you’ve had recent UTIs, and how quickly symptoms developed. For men, the exam may include a check for an enlarged prostate.
A urinalysis is the first lab test, checking your urine sample for white blood cells and blood, both signs of infection. A urine culture follows, which identifies the specific bacteria causing the infection and helps determine which antibiotics will work against it. Blood tests may also be ordered to check for signs of a more serious or widespread infection and to assess how well your kidneys are functioning.
Imaging isn’t always necessary, but it may be used if your doctor suspects a blockage, a structural problem, or if you aren’t improving with treatment. A CT scan is the most common choice, though ultrasound, MRI, or a specialized scan using a small amount of radioactive material can also be used depending on the situation.
Treatment and What to Expect
Antibiotics are the standard treatment. For uncomplicated cases, you’ll take oral antibiotics at home, and most people start feeling noticeably better within 48 to 72 hours. The full course of antibiotics typically lasts 7 to 14 days, and finishing the entire course matters even after symptoms improve.
Some situations call for hospital treatment rather than at-home care. You’ll likely need to be admitted if you can’t keep oral antibiotics down due to vomiting, if you’re pregnant, if there’s a blockage in your urinary tract, or if you’re showing signs of sepsis (a dangerous whole-body response to infection). People with significant underlying conditions like diabetes, immunosuppression, or sickle cell disease may also need inpatient care. Severe pain that doesn’t respond to standard treatment or unreliable access to follow-up care are additional reasons doctors may recommend hospitalization.
What Happens if It Goes Untreated
A kidney infection that isn’t treated promptly can lead to serious complications. The most dangerous is sepsis, where bacteria enter the bloodstream and trigger a widespread inflammatory response that can damage organs and become life-threatening. Repeated or severe kidney infections can also cause permanent scarring of kidney tissue, which over time may impair kidney function.
Kidney stones can both cause and complicate kidney infections. A stone that blocks urine flow creates a pocket of stagnant urine where bacteria thrive, and treating the infection without addressing the blockage often leads to recurrence.
Reducing Your Risk
Staying well hydrated is commonly recommended for preventing urinary tract infections, including kidney infections. The reasoning is straightforward: more fluid means more frequent urination, which flushes bacteria from the urinary tract before they can establish an infection. While this advice is standard, the scientific evidence supporting a specific daily water intake for UTI prevention is surprisingly limited. A systematic review of studies examining fluid intake of 1.5 liters or more per day found inconsistent results.
What does clearly help is treating bladder infections early, before bacteria have a chance to travel to the kidneys. If you’re prone to UTIs, pay attention to early symptoms like burning with urination or increased urgency, and seek treatment quickly. For people with structural risk factors like vesicoureteral reflux or recurrent kidney stones, managing those underlying conditions is the most effective way to prevent repeated infections.