Kidney infections happen when bacteria travel up through the urinary tract, moving from the urethra into the bladder and then climbing higher through the tubes (ureters) that connect the bladder to the kidneys. This ascending path is by far the most common route. The bacteria usually originate from the digestive tract, and in roughly 85% of cases, the specific culprit is E. coli, a bacterium that normally lives in the intestines without causing harm but triggers serious infection once it reaches the kidneys.
How Bacteria Reach the Kidneys
The journey typically starts at the urethra, the short tube that carries urine out of your body. Bacteria from the skin around the anus or genitals can migrate into the urethra and travel up to the bladder. At this stage, you have a bladder infection (commonly called a UTI). If the infection isn’t cleared, bacteria continue climbing through the ureters into one or both kidneys, where they invade kidney tissue and trigger inflammation, pain, and fever.
In rare cases, bacteria from an infection elsewhere in your body can reach the kidneys through the bloodstream. This is far less common and typically only happens when there’s already an underlying problem, such as a blockage in the urinary tract. There’s little evidence that bacteria reach the kidneys through the lymphatic system.
Why Women Get Kidney Infections More Often
Women develop urinary tract infections, and by extension kidney infections, at significantly higher rates than men. The reason is largely anatomical. A woman’s urethra is much shorter than a man’s, meaning bacteria have a shorter distance to travel before reaching the bladder. The urethra is also positioned close to both the anus and the vagina, two areas that harbor bacteria. Sexual activity can push bacteria toward or into the urethra, compounding the risk.
Men have a built-in advantage: the urethral opening sits at the tip of the penis, far from the bladder, and secretions from the prostate gland have bacteria-killing properties. That doesn’t make men immune, but it makes infections considerably less frequent.
After menopause, women face an additional vulnerability. The drop in protective hormones changes the vaginal environment in ways that make bacterial colonization easier, raising the likelihood of infections that can ascend to the kidneys.
Structural Problems That Trap Bacteria
Anything that prevents urine from flowing freely out of the body gives bacteria more time to multiply and travel upward. Kidney stones, enlarged prostates, and narrowed ureters can all create partial blockages that let urine pool in the bladder or ureters.
A condition called vesicoureteral reflux also increases risk. Normally, a valve-like mechanism prevents urine from flowing backward from the bladder toward the kidneys. When that valve doesn’t close properly, urine can carry bacteria back up the ureters and into the kidneys. This can be something you’re born with (a faulty valve) or something that develops later because the bladder doesn’t empty properly, whether due to a tissue blockage, nerve damage, or narrowed muscles at the bladder outlet.
Pregnancy Changes the Urinary Tract
Pregnancy creates a temporary but significant shift in how the urinary system works. Rising progesterone levels cause the ureters to relax and widen, while the growing uterus physically compresses them. Together, these changes leave more urine sitting in the bladder and ureters instead of draining efficiently. That stagnant urine gives bacteria an opportunity to multiply and climb toward the kidneys. The same hormonal changes can also cause urine to flow backward from the bladder into the ureters, mimicking the reflux problem described above. This is why pregnant women are routinely screened for urinary bacteria even when they have no symptoms.
How Diabetes Weakens Kidney Defenses
People with diabetes face a higher risk of kidney infections, and the explanation goes beyond the conventional idea that extra sugar in the urine feeds bacteria. Research published in The Journal of Clinical Investigation revealed a more fundamental problem: the kidneys have specialized cells in their collecting tubes that defend against infection by acidifying urine and releasing natural germ-killing proteins into the urinary stream. These defense proteins depend on insulin signaling to be produced at normal levels.
In type 2 diabetes, where the body resists insulin’s effects, production of these antimicrobial proteins drops. With weakened chemical defenses, bacteria that reach the kidney’s inner tubes can latch onto the lining, multiply inside the cells, and trigger the intense inflammatory response recognized as a kidney infection. Studies found that people with diabetes had lower concentrations of these protective proteins, and the worse their blood sugar control (measured by hemoglobin A1c), the lower the protein levels fell.
Other Factors That Raise Your Risk
- Catheter use. A tube inserted into the bladder provides a direct highway for bacteria, bypassing the body’s normal barriers.
- Weakened immune system. Conditions or medications that suppress immune function make it harder for the body to fight off bacteria before they spread.
- Previous UTIs. A history of bladder infections signals that bacteria have already found a successful route into the urinary tract, and repeat episodes increase the chance one will eventually reach the kidneys.
- Urinary tract surgery or procedures. Any instrumentation of the urinary tract can introduce bacteria or temporarily disrupt the normal flow of urine.
Symptoms That Distinguish a Kidney Infection
A bladder infection typically causes burning with urination, frequent urges to pee, and cloudy or strong-smelling urine. A kidney infection layers on more serious symptoms: fever, chills, nausea or vomiting, and pain in the back or side, usually just below the ribs on one side. The pain can be dull and constant or sharp. Some people also notice blood in their urine.
These symptoms tend to come on quickly, often within a day or two of a worsening bladder infection, though some people develop kidney infections without ever noticing bladder symptoms first.
How a Kidney Infection Is Diagnosed
Doctors typically combine a physical exam with lab tests. A urinalysis checks for white blood cells and blood in the urine, both signs of infection. A urine culture identifies the specific bacteria causing the problem and helps determine which antibiotics will work against it. Blood tests may be ordered to assess how well the kidneys are functioning and to check for signs that the infection has become severe or entered the bloodstream.
Imaging tests like ultrasound or CT scans aren’t always necessary but may be used if the doctor suspects a blockage, kidney stone, or structural abnormality that contributed to the infection. These scans can reveal swelling in the kidney, abscesses, or obstructions that need separate treatment to prevent the infection from recurring.