Kidney infections are almost always caused by bacteria that travel upward from the bladder through the tubes (ureters) connecting it to the kidneys. A single species, E. coli, is responsible for roughly 90% of uncomplicated cases. But the reasons bacteria reach the kidneys in the first place vary widely, from anatomical blockages and hormonal shifts to medical devices and immune system problems.
How Bacteria Reach the Kidneys
The vast majority of kidney infections start as lower urinary tract infections. Bacteria colonize the urethra, move into the bladder, and then climb upward. E. coli strains that cause kidney infections are especially well-equipped for this journey. They produce tiny hair-like structures on their surface that grip the walls of the urinary tract, helping them resist the downward flow of urine. They also have whip-like tails (flagella) that propel them upward. Once these bacteria reach the kidney, they switch to a different attachment mechanism that locks onto kidney tissue directly.
These bacteria can also interfere with the normal rhythmic squeezing of the ureters, which ordinarily helps push urine (and any hitchhiking bacteria) downward toward the bladder. By disrupting that muscular activity, bacteria essentially disable one of the body’s key defenses against ascending infection.
In rare cases, bacteria reach the kidneys through the bloodstream rather than climbing up from the bladder. Staph bacteria, for example, can seed the kidneys after establishing an infection elsewhere in the body, like a skin wound or the heart. Certain fungal infections can also reach the kidneys this way. But blood-borne kidney infections are uncommon compared to the ascending route.
Kidney Stones and Other Blockages
Anything that blocks the normal flow of urine creates a prime environment for infection. Kidney stones are the most common culprit. When a stone lodges in a ureter, urine pools behind it, giving bacteria a stagnant reservoir to multiply in. The relationship goes both directions: infections can promote stone formation, and stones can promote infection. Certain bacteria break down urea into ammonia, which makes urine more alkaline and encourages new stones to form on top of existing ones.
Bacteria also build protective colonies called biofilms on the surface of stones, making them harder for the immune system (and antibiotics) to clear. Infections in the setting of ureteral stones occur in 4% to 34% of patients based on urine culture results, and the combination is far more dangerous than either problem alone. In one study of patients with septic shock from a urinary source, those with an anatomical obstruction had a hospital mortality rate of 27.3%, compared to 11.2% in those without one.
An enlarged prostate can cause similar problems in men by compressing the urethra and preventing complete bladder emptying. Any structural abnormality that causes urine to sit in the urinary tract longer than it should raises the risk.
Female Anatomy and Sexual Activity
Women develop kidney infections far more often than men, largely because of anatomy. The female urethra is much shorter, giving bacteria a shorter path to the bladder. From there, ascending infection to the kidneys follows the same route regardless of sex.
Sexual intercourse can push bacteria from the genital area into the urethra. Using spermicides alters the normal bacterial balance in the vaginal area, potentially allowing harmful strains to flourish. A history of recurrent bladder infections is one of the strongest predictors of eventually developing a kidney infection, since each episode gives bacteria another opportunity to ascend.
Pregnancy
Pregnancy creates a near-perfect setup for kidney infection through several overlapping changes. The bladder loses muscle tone and doesn’t empty completely, allowing urine to flow backward toward the kidneys. The ureters and the collecting system of the kidneys physically widen during pregnancy (a condition called physiologic hydronephrosis), which makes it easier for bacteria to travel upward.
The chemistry of urine also shifts. Urine becomes more alkaline during pregnancy, and it contains more sugar, both of which help bacteria grow faster. Rising estrogen levels have been shown to directly accelerate the growth of E. coli strains that cause kidney infections. These factors together explain why pregnant women with untreated bladder infections are at significantly higher risk of progressing to a kidney infection than non-pregnant women with the same bacteria in their urine.
Diabetes and Weakened Immunity
Poorly controlled diabetes is one of the strongest risk factors for severe kidney infections. High blood sugar impairs the immune system’s ability to fight off bacteria, and sugar in the urine serves as fuel for bacterial growth. Diabetes also damages small blood vessels in the kidneys, which slows the removal of waste products from tissue and may contribute to more aggressive forms of infection.
The most dangerous example is a condition called emphysematous pyelonephritis, where bacteria in the kidney actually ferment glucose and produce gas pockets inside the tissue. About 72% of patients with this condition have poorly controlled blood sugar. It occurs most often in women with diabetes and carries a much higher mortality rate than typical kidney infections.
Other conditions that suppress the immune system raise similar risks. Organ transplant recipients on immunosuppressive medications, people undergoing chemotherapy, and those with HIV are all more vulnerable to kidney infections and more likely to develop complications from them.
Urinary Catheters and Medical Procedures
Urinary catheters are one of the most common hospital-related causes of kidney infection. According to the CDC, the single most important risk factor for catheter-associated urinary tract infections is prolonged catheter use. The tube bypasses the body’s natural barriers and gives bacteria a direct highway into the bladder. From there, infection can ascend to the kidneys just as it would in any other case. Bacteria form biofilms on the catheter surface, making these infections particularly stubborn to treat.
Other procedures involving the urinary tract, such as cystoscopy (where a camera is inserted into the bladder) or surgery on the urinary system, can also introduce bacteria.
Reflux in Children
In children, one of the most important causes of kidney infection is vesicoureteral reflux (VUR), a condition where urine flows backward from the bladder into the ureters and kidneys. About 1 in 3 children who develop a urinary tract infection with fever turn out to have VUR. The backward flow of urine carries bacteria up to the kidneys far more easily than normal anatomy would allow, and repeated infections can scar kidney tissue over time. Most children with mild VUR outgrow it as their urinary tract matures, but more severe cases may need surgical correction to prevent ongoing kidney damage.
Which Bacteria Are Involved
E. coli dominates the list, accounting for about 95% of uncomplicated kidney infections and 77% of complicated ones. The remainder is split among several less common organisms. Klebsiella pneumoniae is the second most frequent cause at roughly 5% of all cases, and it’s more common in complicated infections (those involving structural abnormalities, catheters, or compromised immune systems). Enterococcus, Pseudomonas, and Proteus each account for a small percentage and tend to appear in hospital-acquired or complicated infections rather than straightforward community cases.
The distinction matters because complicated infections are more likely to involve antibiotic-resistant bacteria. When kidney infections develop in people with catheters, diabetes, structural abnormalities, or recent hospital stays, the range of possible bacteria widens considerably and treatment becomes less predictable.