Kidney infections almost always start as a bladder infection that travels upward. Bacteria enter the urethra, colonize the bladder, and then climb through the ureters into one or both kidneys. This ascending route is the most common pathway, and the bacterium responsible in 75 to 95 percent of cases is E. coli, a normal resident of your digestive tract that causes problems when it reaches places it doesn’t belong.
How Bacteria Reach the Kidneys
The process typically begins outside the body. Bacteria from the bowel colonize the skin around the urethra, then work their way inside. Once in the bladder, they can multiply and trigger a lower urinary tract infection, the kind that causes burning during urination and a constant urge to go. If that infection isn’t cleared, bacteria continue upward through the ureters, the narrow tubes connecting each kidney to the bladder, and settle into kidney tissue.
In rare cases, bacteria reach the kidneys through the bloodstream instead. This happens when an infection elsewhere in the body, such as a heart valve infection or a skin infection from intravenous drug use, sends bacteria circulating through the blood that then lodge in the kidneys. Bloodborne kidney infections are far less common and tend to involve different bacteria, particularly Staphylococcus species rather than E. coli.
Why Women Get Kidney Infections More Often
The female urethra averages about 3 centimeters long. The male urethra is several times that length. This difference alone gives bacteria a much shorter distance to travel before reaching the bladder in women, and it explains why urinary tract infections of all kinds are dramatically more common in women. The opening of the urethra also sits closer to the anus in women, making it easier for bowel bacteria to migrate to the urinary tract in the first place.
Sexual activity can push bacteria toward or into the urethra, which is one reason kidney infections spike in sexually active women. Pregnancy adds further risk because the growing uterus can compress the ureters, slowing urine flow and giving bacteria more time to establish themselves.
Blockages That Trap Bacteria
Anything that slows or stops the flow of urine creates conditions for infection. Kidney stones are a major culprit. When a stone partially or fully blocks a ureter, urine pools behind it, and bacteria that would normally be flushed out can multiply in that stagnant urine. Kidney infections tied to stones are notoriously stubborn. The infection typically will not fully resolve until the stone is removed.
An enlarged prostate can have the same effect in men, compressing the urethra and preventing the bladder from emptying completely. Structural abnormalities present from birth also play a role. One of the most significant is vesicoureteral reflux, a condition where the connection between the ureter and bladder doesn’t close properly, allowing urine to flow backward from the bladder up toward the kidney. This backwash carries bacteria directly into the kidney and is a common cause of kidney infections in children.
Why Diabetes Raises Your Risk
People with diabetes develop kidney infections at higher rates, and the reason goes beyond the traditional explanation that sugar in the urine feeds bacteria. Research published in The Journal of Clinical Investigation identified a more fundamental problem: insulin resistance impairs the kidney’s own defense system.
Cells in the kidney’s collecting ducts produce natural antimicrobial proteins that kill bacteria on contact. The production of these proteins depends on normal insulin signaling. In type 2 diabetes, where cells respond poorly to insulin, the output of at least two key antimicrobial proteins drops significantly. Studies in both mice and humans confirmed that people with diabetes had lower concentrations of these protective proteins, and the worse their blood sugar control (measured by hemoglobin A1c), the lower those levels fell. Even in animal models that had insulin resistance without high blood sugar, bacterial defenses were compromised, confirming that the problem is the faulty insulin signaling itself, not just excess glucose.
Other Factors That Increase Risk
Catheters provide a direct highway for bacteria into the bladder, bypassing the body’s normal barriers. The longer a catheter stays in place, the higher the infection risk. A weakened immune system from any cause, whether from medication, illness, or age, also makes it harder for the body to fight off bacteria before they reach the kidneys.
Previous urinary tract infections are one of the strongest predictors of future ones. Some people are simply more susceptible due to the way their cells interact with bacteria, making colonization easier each time. Holding urine for long periods or not drinking enough fluids can also contribute by reducing the flushing action that normally keeps bacterial counts low.
How a Kidney Infection Feels
The hallmark symptom is pain in your side or lower back, on the side of the affected kidney. This flank pain, combined with a fever and evidence of infection in your urine, is the combination that distinguishes a kidney infection from a simple bladder infection. Most people also experience chills, nausea, or vomiting. Some develop abdominal pain or a rapid heartbeat.
What catches some people off guard is that the typical bladder infection symptoms, the burning and urgency, are actually absent in about 20 percent of kidney infection cases. You can have a kidney infection without ever feeling like you had a bladder infection first. Fever is common but not universal either, so the combination of back or side pain with general illness is the signal to take seriously.
How It’s Identified and Treated
A urine sample showing bacteria and white blood cells, paired with flank pain and tenderness, is usually enough for a diagnosis. Imaging isn’t needed in straightforward cases. It becomes important when there’s concern about a stone causing the infection, when symptoms don’t improve within 48 to 72 hours of treatment, or when there are signs of a more serious complication like sepsis.
Most uncomplicated kidney infections are treated with antibiotics and resolve within one to two weeks. The key variable is whether anything is blocking urine flow. If a stone or structural problem is trapping bacteria, antibiotics alone may not be enough, and the underlying blockage needs to be addressed. Kidney infections that go untreated or that develop in people with obstructions can lead to permanent scarring of kidney tissue, so the combination of flank pain, fever, and feeling systemically unwell is not something to wait out.