The clearest sign of a kidney infection is a combination of fever, chills, and pain in your back or side, typically just below your rib cage. A simple bladder infection causes burning and frequent urination, but once that infection travels upward to one or both kidneys, you develop whole-body symptoms like nausea, vomiting, and fatigue that a bladder infection alone doesn’t cause. If you’re experiencing these together, a kidney infection is likely and needs prompt treatment.
Key Symptoms of a Kidney Infection
Kidney infections share some symptoms with bladder infections but add a distinct set of their own. The full picture typically includes:
- Fever above 102°F and chills or shaking
- Pain in your back, side (flank), or groin
- Nausea or vomiting
- Cloudy, dark, bloody, or foul-smelling urine
- Frequent or painful urination
- General body aches and fatigue
The fever and flank pain are the big differentiators. A bladder infection can give you a low-grade fever, but kidney infections tend to push temperatures higher and come with shaking chills that make you feel genuinely sick. You may also notice pus in your urine, which can make it look especially cloudy.
Where Kidney Pain Actually Is
People often assume kidney pain is the same as lower back pain. It isn’t. Your kidneys sit higher than most people think, behind your stomach and tucked under your lower rib cage on either side of your spine. Kidney infection pain is typically felt in your flank, the area between your lower ribs and your hip on one or both sides. It tends to feel deep, not surface-level, and it can radiate forward into your abdomen or down toward your groin.
Back pain from a muscle strain or spinal issue, by contrast, usually centers over your spine and may radiate down your legs. Kidney pain stays to the side, feels deeper, and doesn’t improve or worsen when you shift positions the way muscular back pain does. If pressing on the area just below your ribs on one side makes you wince, that’s a strong clue.
How It Differs From a Bladder Infection
Most kidney infections start as bladder infections (cystitis) that weren’t treated or didn’t clear fully. Bladder infections cause burning when you urinate, a constant urge to go, and pain above your pubic bone or in your lower abdomen. They’re uncomfortable but localized. You generally feel fine otherwise.
When the infection climbs from your bladder through the ureters and into the kidney itself, the symptoms escalate. You develop a high fever, chills, flank pain, and nausea or vomiting. You may feel wiped out, with general body aches and damp or clammy skin. The shift from “annoying urinary symptoms” to “I feel sick all over” is the hallmark of a bladder infection becoming a kidney infection. If you were recently treated for a UTI and these symptoms appear, that’s a strong signal the infection has spread upward.
Symptoms in Older Adults and Children
Kidney infections don’t always follow the textbook pattern. In older adults, especially those with dementia or cognitive decline, the most prominent symptom can be sudden, severe confusion rather than the classic fever and flank pain. Increased agitation, withdrawal, or a dramatic change in behavior that comes on quickly may be the only visible clue. Because the person may not be able to describe their symptoms, caregivers should watch for these behavioral shifts and consider a urinary tract or kidney infection as a possible cause.
Children under two years old may show nothing beyond a high fever. They won’t be able to tell you about pain, and their symptoms may look like feeding difficulty, irritability, or poor weight gain. A persistent unexplained fever in a young child warrants testing for a kidney infection.
Who Is at Higher Risk
Kidney infections can happen to anyone, but certain factors raise your odds. Women get them more frequently than men because the urethra is shorter, giving bacteria a shorter path to the bladder and kidneys. Being pregnant increases risk further because the growing uterus can compress the ureters and slow urine drainage, making it easier for bacteria to multiply. Women who had gestational diabetes face additional long-term kidney vulnerability: research from the NIH found they were more than three times as likely to show early signs of kidney strain compared to women without gestational diabetes.
Diabetes in general raises risk, as does any condition that weakens your immune system. A history of kidney stones matters too, because stones can block urine flow and create a breeding ground for infection. If you’ve had frequent UTIs, structural abnormalities in your urinary tract, or recently had a catheter, your risk is also elevated.
What Happens When You Get Checked
A urine test is the first step. It checks for bacteria, white blood cells, and sometimes blood in your urine, all of which point toward infection. A urine culture identifies the specific bacteria involved, which helps determine the right antibiotic. Blood tests may be added to check whether the infection has entered your bloodstream.
Most people with a kidney infection don’t need imaging. A CT scan of the abdomen and pelvis with contrast is reserved for specific situations: if your fever hasn’t broken after 72 hours of antibiotics, if you have diabetes or a weakened immune system, if there’s a history of kidney stones, or if your symptoms are unusually severe or complex. The scan looks for complications like abscesses or blockages that would require a different treatment approach. For a straightforward kidney infection in an otherwise healthy person, antibiotics and a urine test are usually all that’s needed.
What Treatment Feels Like
A kidney infection is treated with antibiotics, typically for 7 to 14 days depending on severity. Most people can take oral antibiotics at home. You should start feeling noticeably better within 48 to 72 hours, though it’s important to finish the full course even after symptoms improve. Stopping early is one of the most common reasons infections come back.
If your infection is severe, if you can’t keep fluids or medication down because of vomiting, or if you’re pregnant, you may need to be treated in a hospital with IV fluids and antibiotics until you stabilize enough to switch to oral medication. Drink plenty of water throughout recovery. Rest as much as your body is asking for, especially in the first few days when fever and fatigue are at their worst.
Warning Signs That Need Emergency Care
A small percentage of kidney infections progress to a dangerous condition called urosepsis, where the infection enters the bloodstream and triggers a body-wide inflammatory response. This is a medical emergency. The warning signs include a rapid heart rate or heart palpitations, difficulty breathing, a weak pulse, inability to urinate, and feeling confused or disoriented. If you’ve been running a fever and suddenly feel significantly worse, especially with any combination of these symptoms, go to an emergency room. Sepsis can escalate quickly, and early treatment dramatically improves outcomes.