Kidney infections are caused by bacteria that travel upward from the bladder into one or both kidneys. The bacterium E. coli is responsible for roughly 80% of cases in women and 70% in men, with the remainder caused by other gut-related bacteria. Unlike a simple bladder infection, a kidney infection can become serious quickly, so understanding what triggers one helps you recognize and reduce your risk.
How Bacteria Reach the Kidneys
Nearly all kidney infections start as lower urinary tract infections. Bacteria, usually from the digestive tract, first colonize the urethra, then move into the bladder. If a bladder infection isn’t cleared by your immune system or treated with antibiotics, those bacteria can continue climbing up the ureters, the two thin tubes connecting the bladder to the kidneys.
Once bacteria reach the kidney, they settle into the tissue of the renal pelvis, the funnel-shaped area where urine collects before draining downward. The infection triggers inflammation, swelling, and the hallmark symptoms: flank pain, fever, chills, and painful urination. This ascending route, from urethra to bladder to kidney, is by far the most common pathway. In rare cases, bacteria from an infection elsewhere in the body can reach the kidneys through the bloodstream, but this is uncommon outside of hospital settings.
The Bacteria Behind It
E. coli dominates as the primary cause because it naturally lives in the intestines and can easily reach the urinary opening. It also has specialized surface structures that help it cling to the lining of the urinary tract, making it harder for the body to flush out. After E. coli, the next most common culprits are Klebsiella, Proteus, Pseudomonas, and certain strains of Enterococci and Staphylococci. In people who are hospitalized, have diabetes, or have been on prolonged antibiotic courses, a type of yeast called Candida can also cause kidney infections.
Urine Flowing the Wrong Direction
Normally, urine flows in one direction only: from the kidneys down to the bladder. Some people have a condition where urine flows backward from the bladder up toward the kidneys, known as vesicoureteral reflux. This creates a direct channel for infected urine to reach the kidneys, allowing a simple bladder infection to progress to a kidney infection rapidly. Vesicoureteral reflux is most often diagnosed in children but can persist into adulthood. The reflux itself doesn’t damage the kidneys if the urine is sterile, but when bacteria are present, the risk of kidney infection rises significantly.
Blockages That Trap Bacteria
Anything that slows or blocks the normal flow of urine gives bacteria more time to multiply and establish an infection. The most common obstructions include:
- Kidney stones: These can lodge in a ureter and partially or completely block urine drainage, creating a pool of stagnant urine where bacteria thrive.
- Enlarged prostate: In men, a swollen prostate gland can compress the urethra, making it harder to fully empty the bladder. Residual urine becomes a breeding ground for bacteria.
- Structural abnormalities: Narrowing of the ureters or other anatomical differences present from birth can impair drainage and increase infection risk throughout life.
When urine can’t flow freely, bacteria that would normally be flushed out with urination instead accumulate and migrate upward.
Why Women Get Kidney Infections More Often
Women develop kidney infections far more frequently than men, primarily because of anatomy. The female urethra is significantly shorter than the male urethra, which means bacteria have a much shorter distance to travel from the outside of the body to the bladder. The urethral opening is also closer to the rectum, where E. coli is abundant. Sexual activity can push bacteria toward and into the urethra, and using spermicides or diaphragms alters the normal bacterial balance in the vaginal area, making colonization by harmful bacteria more likely.
Pregnancy and Kidney Infection Risk
Pregnancy creates a near-perfect setup for kidney infections. As the uterus grows, it physically compresses the ureters, particularly the right one. The right ureter crosses over a major blood vessel at an angle that makes it especially vulnerable to pressure from the expanding uterus. The left ureter gets some protection from the sigmoid colon. Constipation, which is common during pregnancy, worsens the compression by pushing the uterus further to the right.
These changes cause the ureters and the collecting areas of the kidneys to dilate, a condition that occurs to some degree in most pregnancies. The dilated system holds more urine and drains more slowly, and this stagnation dramatically increases infection risk. Studies suggest the risk of developing a kidney infection can be as high as 40% once the urinary system is significantly dilated during pregnancy. Hormonal shifts also relax the smooth muscle in the ureters, further reducing the ability to push urine downward efficiently.
How Diabetes Weakens Kidney Defenses
People with diabetes face a higher risk of kidney infections, and the reason goes beyond just having sugar in the urine. While glucose in urine does create a friendlier environment for bacterial growth, research published in the Journal of Clinical Investigation points to a more fundamental problem: insulin resistance disables part of the kidney’s built-in immune defense.
Specialized cells in the kidney’s collecting ducts produce natural antimicrobial compounds that kill bacteria on contact. The production of these compounds depends on normal insulin signaling. In type 2 diabetes, where cells become resistant to insulin, these antimicrobial defenses are significantly suppressed. In animal studies, when insulin signaling was knocked out in these kidney cells, their urine lost the ability to suppress the growth of E. coli, while urine from normal animals could. The kidney tissue itself looked structurally normal, meaning the problem isn’t damage to the kidney but a silent weakening of its ability to fight off invading bacteria.
Urinary Catheters and Hospital-Acquired Infections
Urinary catheters are one of the most significant causes of kidney infections in hospital and long-term care settings. A catheter is a thin tube inserted through the urethra into the bladder to drain urine, and it essentially creates a highway for bacteria to bypass the body’s natural defenses.
The risk of bacteria entering the bladder increases by 3 to 7% for each day a catheter remains in place. In intensive care units, where infection rates are three to five times higher than in other hospital areas, catheter-related urinary infections occur at a rate of roughly 8 per 1,000 catheter days. Biofilm, a protective layer of bacteria, begins forming on the catheter surface within minutes of insertion. This biofilm shields bacteria from both antibiotics and the immune system, allowing infection to persist and potentially ascend to the kidneys.
Catheters also cause physical irritation to the urinary tract lining, stripping away a protective coating that normally prevents bacteria from attaching. The body’s inflammatory response to the catheter itself deposits proteins on the tube’s surface that certain bacteria are specifically adapted to grab onto, compounding the problem.
Other Factors That Raise Your Risk
A weakened immune system from any cause, whether from medications that suppress immunity, HIV, or chronic illness, reduces your ability to fight off urinary bacteria before they reach the kidneys. Prior kidney infections also increase the likelihood of future ones, as scarring from earlier infections can subtly alter drainage patterns. People who have had urinary tract surgery or procedures are at elevated risk in the weeks following, as instrumentation can introduce bacteria directly.
Age plays a role as well. Older adults face higher rates of kidney infections due to a combination of factors: incomplete bladder emptying becomes more common, immune function gradually declines, and conditions like enlarged prostate or pelvic organ prolapse create structural changes that favor bacterial growth. Extended antibiotic use can also paradoxically increase risk by killing off protective bacteria and allowing resistant or opportunistic organisms to take hold.