Kidney infections almost always start as bladder infections that travel upward. Bacteria from the digestive tract migrate through the urethra into the bladder, and if the infection isn’t cleared, those bacteria climb the ureters (the tubes connecting each kidney to the bladder) and reach the kidneys. This ascending route is by far the most common pathway, though in rare cases bacteria can reach the kidneys through the bloodstream from an infection elsewhere in the body.
Understanding how kidney infections develop helps you recognize the situations that raise your risk and the warning signs that a simple UTI may be getting worse.
How a Bladder Infection Becomes a Kidney Infection
The process starts with bacteria, most commonly E. coli, that normally live in the gut. These bacteria can colonize the skin around the urethra and work their way into the bladder. Once there, they multiply in the urine and attach to the bladder wall, causing the burning, urgency, and frequency of a typical UTI. If the immune system and normal urine flow don’t flush the bacteria out, they can travel up the ureters into the kidney tissue itself. This is called pyelonephritis.
The good news: progression from bladder to kidneys is relatively uncommon. In studies tracking people with uncomplicated bladder infections who didn’t receive antibiotics, only about 0.4% to 2.6% went on to develop a kidney infection. Your body clears most bladder infections before they can spread. But certain conditions tilt the odds significantly, making that upward journey much easier for bacteria.
The Bacteria Behind It
E. coli accounts for roughly 31% of urinary tract infections and is the dominant cause of kidney infections. These aren’t the same strains that cause food poisoning. They’re specialized varieties that have surface structures allowing them to grip the lining of the urinary tract and resist being flushed out by urine flow. Other bacteria that cause kidney infections include Klebsiella (about 23% of urinary infections), Pseudomonas (16%), and smaller contributions from Enterococcus, Streptococcus, Staphylococcus, and Proteus species.
Risk Factors That Open the Door
Female Anatomy
Women get kidney infections far more often than men, and the reason is straightforward: the female urethra is only about 3 to 4 centimeters long, compared to roughly 20 centimeters in men. That shorter distance gives bacteria a much easier path to the bladder. The urethra also sits close to the anus, where gut bacteria are always present. From the bladder, the route to the kidneys is the same for everyone.
Urinary Blockages
Anything that slows or stops the normal flow of urine creates a pool where bacteria can multiply. Kidney stones are the classic example. A stone lodged in a ureter traps urine behind it, turning a manageable number of bacteria into a full-blown infection quickly. An enlarged prostate in men can have the same effect by compressing the urethra and preventing the bladder from emptying completely. Ureteral strictures (narrowed sections of the tubes between the kidneys and bladder) also slow drainage and can lead to infection and, if untreated, kidney damage.
Catheters
A urinary catheter is a tube inserted through the urethra to drain the bladder, commonly used during hospital stays or for people with certain chronic conditions. The catheter creates a direct highway for bacteria to enter the urinary tract, bypassing the body’s normal defenses. According to the CDC, the single most important risk factor for a catheter-associated urinary tract infection is how long the catheter stays in place. Every additional day increases the chance of bacterial colonization, and those infections can reach the kidneys.
Vesicoureteral Reflux
Normally, urine flows in one direction: from the kidneys down to the bladder. In vesicoureteral reflux (VUR), a faulty valve at the junction of the ureter and bladder allows urine to flow backward toward the kidneys. This is most common in children and is often something they’re born with. When urine reverses direction, it can carry bacteria from the bladder directly into the kidneys, which is why children with VUR are significantly more prone to recurrent kidney infections. Some children outgrow the condition as the ureter matures, but others need surgical correction.
Secondary VUR can also develop from a blockage in the bladder or urethra, or from nerve problems that prevent the bladder from contracting normally. In either case, urine that can’t exit the body efficiently backs up and creates an environment where bacteria thrive.
Weakened Immune System
Conditions like diabetes, HIV, or medications that suppress the immune system reduce your body’s ability to fight off bacteria at every stage. A bladder infection that a healthy immune system might resolve on its own can progress rapidly to the kidneys when those defenses are compromised.
Why Pregnancy Raises the Risk
Pregnancy creates a perfect storm of changes that make kidney infections more likely. The hormonal shifts of pregnancy relax the smooth muscle of the ureters and bladder, causing the entire upper urinary tract to dilate. A dilated urinary system can hold an extra 200 to 300 milliliters of urine beyond normal bladder capacity. Up to 90% of pregnant people develop some degree of this dilation, called physiologic hydronephrosis. That pooled urine acts as a reservoir for bacterial growth.
At the same time, pregnancy hormones decrease the tone of the bladder muscle, so the bladder doesn’t contract as forcefully or empty as completely. Higher volumes of residual urine after each trip to the bathroom give bacteria more time to multiply. The growing uterus also physically compresses the ureters, particularly on the right side (about 80% of pregnancy-related urinary dilation occurs on the right, because the uterus naturally tilts that way).
On top of all this, pregnancy increases the amount of glucose and amino acids excreted in the urine, essentially providing extra nutrients for bacteria. Combined with the difficulty of maintaining hygiene as the abdomen grows, these factors make UTIs the most common bacterial infection in pregnancy and increase the chance that a bladder infection will progress to the kidneys.
Symptoms That Signal Kidney Involvement
A bladder infection typically causes pain or burning during urination, frequent urges to urinate, and sometimes cloudy or strong-smelling urine. When the infection reaches the kidneys, the symptoms shift noticeably. You’ll often develop a fever, sometimes with chills. Pain moves to the flank area, meaning the side of your back below the ribs, and it can be intense. Nausea and vomiting are common. Some people also notice blood in their urine.
The key distinction is systemic illness. A bladder infection is uncomfortable but localized. A kidney infection makes you feel sick all over, with fatigue, fever, and sometimes confusion in older adults. If you’ve been dealing with UTI symptoms and suddenly develop a fever or back pain, the infection has likely moved beyond the bladder.
What Happens If It Goes Untreated
A kidney infection that isn’t treated can cause lasting damage to the kidney tissue itself, leading to scarring that reduces the kidney’s ability to filter blood effectively. In severe cases, the bacteria can enter the bloodstream, a condition called sepsis, which is a medical emergency. Sepsis triggers widespread inflammation that can damage organs throughout the body. The Mayo Clinic notes that severe kidney infections can lead to blood poisoning, tissue damage, and in the worst cases, death. This is why kidney infections, unlike many bladder infections, consistently require antibiotic treatment rather than watchful waiting.
Repeated kidney infections, particularly in children with vesicoureteral reflux, can cause cumulative scarring that compromises kidney function over time. Each episode of infection creates a little more damage, which is why identifying and addressing the underlying cause matters as much as treating the acute infection.