Kidney infections are almost always caused by bacteria that enter the urinary tract and travel upward from the bladder to the kidneys. The bacterium E. coli is responsible for about 80% of cases in women and 70% in men. An untreated bladder infection can reach the kidneys in as little as a few days to a week, though the exact timeline depends on your overall health, age, and immune function.
How Bacteria Reach the Kidneys
The vast majority of kidney infections follow what’s called an ascending path. Bacteria first colonize the urethra (the tube you urinate through), then move into the bladder, causing a bladder infection. If that infection isn’t cleared, bacteria can continue climbing through the ureters, the narrow tubes connecting the bladder to each kidney. Once bacteria reach the kidney tissue itself, the infection becomes significantly more serious.
In rare cases, bacteria reach the kidneys through the bloodstream rather than climbing up from the bladder. This tends to involve different organisms, particularly staph bacteria, which can travel from an infection elsewhere in the body and seed in the kidneys. This route is uncommon but more likely in people with bloodstream infections or compromised immune systems.
The Bacteria Behind It
E. coli dominates as the primary cause, but it’s not the only player. In older adults (over 55) and in men, other bacteria like Klebsiella and Enterococcus species take on a larger role. Men in general have a more varied mix of bacteria responsible for kidney infections, with E. coli accounting for roughly 71% to 74% of cases rather than the 80% seen in women.
This matters because the type of bacteria influences how the infection responds to treatment. When kidney infections recur or develop in people with underlying health conditions, the responsible bacteria are more likely to be resistant to common antibiotics.
Why Women Get Kidney Infections More Often
Women are significantly more prone to kidney infections, and the reason is straightforward anatomy. The female urethra is much shorter than the male urethra, giving bacteria a shorter distance to travel to reach the bladder. The urethral opening is also closer to the rectum, where E. coli naturally lives. Sexual activity can push bacteria toward the urethra, and using certain types of birth control (like spermicides) can alter the bacterial balance in ways that favor infection.
Urinary Blockages and Incomplete Emptying
Anything that prevents your bladder from emptying fully creates a pool of stagnant urine where bacteria can multiply. Several common conditions do exactly this.
Kidney stones can physically block a ureter, trapping urine above the obstruction. That stagnant urine becomes an ideal breeding ground for bacteria. In men, an enlarged prostate (benign prostatic hyperplasia) compresses the urethra and obstructs urine flow, leading to poor bladder emptying or even chronic urinary retention. Men with an enlarged prostate are more prone to urinary tract infections for this reason, and those infections can ascend to the kidneys.
Other structural problems that obstruct urine flow, such as tumors, scar tissue from previous surgeries, or congenital abnormalities of the urinary tract, carry the same risk. The common thread is always the same: urine that sits too long gives bacteria time to grow and travel.
Vesicoureteral Reflux
In some people, urine flows backward from the bladder up into the ureters and toward the kidneys. This condition, called vesicoureteral reflux, affects about 1% of the general population but is far more common in children with a family history of the condition (detected in 27% to 36% of siblings and offspring of affected individuals). High voiding pressures and incomplete bladder emptying push urine in the wrong direction, and if that urine carries bacteria, it delivers the infection directly to the kidneys. The association between reflux and febrile urinary tract infections is well established, which is why children with recurrent kidney infections are often screened for this condition.
Pregnancy
Pregnant women face a uniquely elevated risk. Progesterone, which rises dramatically during pregnancy, relaxes the smooth muscle of the ureters and causes them to dilate. At the same time, the growing uterus physically compresses the ureters. These two forces combine to increase residual urine volume in the bladder and create urinary stasis. The result is a higher likelihood of bacteria colonizing the urinary tract and ascending to the kidneys. This is why pregnant women are routinely screened for bacteria in their urine, even without symptoms.
Diabetes and Immune Suppression
People with type 2 diabetes face kidney infection risk from multiple angles. Higher glucose concentrations in urine directly promote the growth of pathogenic bacteria. Elevated glucose levels in the kidney tissue itself create a favorable environment for bacteria to multiply, which is one reason diabetic patients are more susceptible to severe kidney infections. On top of that, diabetes-related nerve damage (autonomic neuropathy) can impair the bladder’s ability to contract and empty properly, reducing the physical flushing of bacteria that normal urination provides.
Any condition that weakens the immune system, including HIV, chemotherapy, long-term steroid use, or organ transplant medications, reduces your body’s ability to fight off bacteria before they reach the kidneys. People with weakened immune systems also face a higher risk of rapid progression from a simple bladder infection to a kidney infection.
Catheters and Medical Devices
Urinary catheters are one of the most common causes of hospital-acquired kidney infections. A catheter provides a direct physical pathway for bacteria to enter the bladder, bypassing the body’s normal defenses. The longer a catheter stays in place, the higher the risk. Other urological procedures or instruments that enter the urinary tract can introduce bacteria in a similar way.
How Quickly It Happens
A bladder infection can escalate into a kidney infection in a matter of days. On average, the window is a few days to about a week for an untreated lower urinary tract infection to ascend to the kidneys. People who are older, immunocompromised, or prone to recurrent infections face faster progression. This is why the symptoms of a bladder infection (burning with urination, frequent urge to go, cloudy urine) deserve prompt attention. Once bacteria reach the kidneys, symptoms shift: fever, flank pain, nausea, and chills typically appear.
How Kidney Infections Are Treated
Antibiotics are the standard treatment. Current guidelines from the Infectious Diseases Society of America recommend shorter courses than were once typical. For most people with a kidney infection who are responding well to treatment, 5 to 7 days of antibiotics is sufficient, depending on the specific medication used. The treatment clock starts from the first day you’re on an effective antibiotic, meaning one that targets the bacteria actually causing your infection.
For kidney infections that have spread to the bloodstream, 7 days of treatment is generally adequate as long as symptoms are improving. Men in whom a prostate infection is suspected may need a longer course of 10 to 14 days. Urine cultures, taken before starting antibiotics, help identify the exact bacteria involved and ensure the right medication is chosen.