Pyelonephritis is a serious bacterial infection of the kidneys that can cause lasting damage if not treated quickly. Understanding this condition helps you recognize its symptoms early and seek the medical care needed to prevent complications.
Understanding Pyelonephritis
Pyelonephritis is the medical term for a kidney infection. This condition occurs when bacteria move from the lower parts of your urinary system up into one or both of your kidneys. Your urinary system works like a drainage network: your kidneys filter waste from your blood and create urine, which then travels down tubes called ureters (the tubes connecting kidneys to the bladder) to your bladder, where it’s stored until you’re ready to urinate. Finally, urine leaves your body through another tube called the urethra (the tube that carries urine out of the body).[1]
Most kidney infections begin as bladder infections. When bacteria enter through the urethra and settle in the bladder, they can multiply and travel upward through the ureters to reach the kidneys. Once bacteria arrive in the kidneys, they trigger inflammation and infection. While bladder infections are quite common and usually not serious, kidney infections are much more concerning because they can permanently scar your kidneys and, in rare cases, spread bacteria into your bloodstream, creating a life-threatening situation.[2]
There are two main forms of this condition. Acute pyelonephritis comes on suddenly and severely, causing intense symptoms that develop quickly. Chronic pyelonephritis refers to long-lasting or recurring kidney infections, though this form is much rarer and tends to happen more often in children or people who have structural problems with their urinary system.[5]
How Common Are Kidney Infections
Kidney infections affect approximately 1 in every 2,000 people in the United States each year. The condition leads to roughly 250,000 doctor visits and 200,000 hospital stays annually in the country.[1][4]
Women are far more likely to develop kidney infections than men. The highest number of cases occurs in otherwise healthy women between the ages of 15 and 29 years old. After this age group, infants and elderly people are the next most commonly affected. A population-based study in the United States found that women experience kidney infections at rates of 15 to 17 cases per 10,000 people annually, while men have only 3 to 4 cases per 10,000 people each year.[7]
The pattern of who gets kidney infections relates directly to anatomy and life circumstances. Women have shorter urethras than men, which means bacteria have a shorter distance to travel to reach the bladder and potentially the kidneys. Pregnancy also increases risk, and kidney infections are notably more common during pregnancy. Men become more susceptible after age 50, usually due to prostate enlargement that can block urine flow.[8]
What Causes Kidney Infections
Bacterial infections are by far the most common cause of pyelonephritis. The bacterium called Escherichia coli, commonly known as E. coli, is responsible for 75 to 95 percent of all cases. This type of bacteria normally lives harmlessly in your intestines and on your skin, but when it enters the urinary tract, it can cause infection.[2]
Other bacteria that can cause kidney infections include Proteus mirabilis, Enterobacter, Staphylococcus, and Klebsiella species. In rare cases, the infection might be caused by Streptococcus agalactiae or Enterococcus species. Viruses can also cause kidney infections, but this is extremely uncommon in people who are otherwise healthy.[1][13]
Bacteria reach the kidneys in two main ways. The most common route is called ascending infection. This happens when bacteria enter through the urethra, multiply in the bladder, and then climb upward through the ureters to reach the kidneys. Less frequently, bacteria traveling through the bloodstream from another part of your body can settle in the kidneys and cause infection there.[2]
Your body normally has defense mechanisms to prevent infections from reaching your kidneys. When you urinate, the flow of urine helps wash out bacteria and other germs before they can cause problems. However, when bacteria multiply faster than your body can remove them, or when there are blockages that prevent complete bladder emptying, bacteria get the opportunity to travel upward and establish an infection in the kidneys.[1]
Risk Factors for Developing Pyelonephritis
Several factors increase your chances of developing a kidney infection. Understanding these risk factors helps you recognize when you might be more vulnerable to this condition.
Your biological sex plays a significant role. Women are at much higher risk than men because they have a shorter urethra, making it easier for bacteria to reach the bladder and potentially travel upward to the kidneys. Sexual activity also increases risk for women, as does the use of spermicide-containing contraceptives.[4][13]
Having a current or recent bladder infection significantly raises your risk of kidney infection. If you’ve had previous urinary tract infections, you’re also more susceptible to future infections that could reach your kidneys. Any condition or situation that prevents you from completely emptying your bladder creates an environment where bacteria can multiply and potentially cause kidney infection.[4]
Physical blockages anywhere in your urinary tract increase your vulnerability. Kidney stones can block the flow of urine, giving bacteria time to multiply. In men, an enlarged prostate can create similar blockages. Women may develop a condition called uterine prolapse, where the uterus drops down and presses against the bladder, interfering with normal urine flow. During pregnancy, pressure on the bladder from the growing baby can also create conditions favorable for infection.[1]
A condition called vesicoureteral reflux (when urine flows backward from the bladder toward the kidneys) creates a pathway for bacteria to reach the kidneys more easily. This condition is more common in children but can affect adults as well.[1]
Certain health conditions make infections more likely or more serious. People with diabetes have higher risks because elevated blood sugar can impair immune function and create conditions that favor bacterial growth. Those with weakened immune systems—whether from HIV infection, cancer treatments, or medications that suppress immunity—are more vulnerable to all types of infections, including kidney infections.[1]
Damage to nerves that control the bladder, such as from spinal cord injury, can prevent complete bladder emptying. People who need urinary catheters or other instruments inserted into their urinary tract also face increased risk because these devices can introduce bacteria directly into the urinary system.[2]
Recognizing the Symptoms
The symptoms of pyelonephritis typically develop rapidly, often appearing within just a few hours or over the course of a day or two after bacteria reach the kidneys. Knowing what to watch for helps you seek treatment quickly, which is crucial for preventing complications.[8]
Fever is one of the hallmark symptoms of kidney infection. The fever is often quite high, frequently exceeding 102°F (38.9°C). Along with fever, you may experience shaking chills that can be quite severe. The fever and chills reflect your body’s immune response fighting the infection.[1][5]
Pain is another key symptom. The characteristic pain of kidney infection occurs in your flank—the area just below your ribs on either side of your back. This flank pain can be intense and may travel around your side toward your lower abdomen. The pain is nearly universal in kidney infections, and its absence should make your doctor consider other possible diagnoses. Some people also experience pain in their lower back or side that feels different from typical back pain.[1][8]
You’ll likely notice changes in your urine and urination. Your urine may appear cloudy or have a pinkish or reddish tint from blood. The urine might smell unusually strong, foul, or fishy. You may feel burning or pain when you urinate, need to urinate more frequently than usual, or feel an urgent need to urinate even when your bladder isn’t full.[1][5]
Digestive symptoms often accompany kidney infections. Many people experience nausea and vomiting. You might lose your appetite and generally feel unwell or achy all over. Some people develop abdominal pain or discomfort in addition to flank pain.[2][5]
Other symptoms can include extreme tiredness, general feelings of illness or malaise, and moist or clammy skin. In some cases, people notice that they’re having daytime or nighttime accidents if they’re normally toilet trained, which can be a sign of infection.[5][6]
It’s important to know that symptoms can vary by age. In older adults, mental confusion may be the most prominent symptom, sometimes even the only noticeable sign of kidney infection. In infants and very young children, symptoms can be quite different from adults—they might simply have poor feeding, vomiting, and fever, without the typical pain complaints that older children and adults would report.[5][6]
People with chronic pyelonephritis may have only mild symptoms or sometimes no noticeable symptoms at all, which makes this form particularly concerning because kidney damage can progress without obvious warning signs.[5]
Preventing Kidney Infections
While you cannot prevent all kidney infections, there are several practical steps you can take to reduce your risk. Many of these strategies focus on preventing bladder infections, since kidney infections most often develop when bladder infections spread upward.
Staying well hydrated is one of the most important preventive measures. Drinking plenty of water throughout the day helps you urinate more frequently, which flushes bacteria out of your urinary tract before they have a chance to multiply and cause infection. Unless your doctor has told you to limit fluids because of kidney, heart, or liver disease, aim to drink enough water so that your urine stays light in color.[1][8]
Don’t hold your urine for long periods. When you feel the urge to urinate, go to the bathroom promptly. Holding urine in your bladder gives bacteria more time to multiply. Try to urinate regularly throughout the day and empty your bladder completely each time you go.[17]
For women, several specific practices can help prevent infections. Always wipe from front to back after using the toilet—this prevents bacteria from the anal area from spreading toward the urethra. Urinate shortly after sexual intercourse, as this helps flush out any bacteria that might have entered the urethra. Keep your genital area clean using mild soap and water. Avoid using feminine hygiene sprays, douches, or other deodorant products in the genital area, as these can irritate the urethra and potentially increase infection risk.[1][17]
Change sanitary pads frequently during menstruation. Some women find that certain types of birth control, particularly spermicide-containing products, increase their risk of urinary infections. If you notice a pattern of infections related to a particular contraceptive method, discuss alternatives with your healthcare provider.[1]
For men, keeping the tip of the penis clean is important for preventing bacteria from entering the urinary tract.[17]
Perhaps most importantly, seek medical attention quickly if you develop symptoms of a bladder infection, such as burning during urination or needing to urinate very frequently. Treating bladder infections promptly prevents them from spreading to the kidneys. Don’t wait to see if symptoms will go away on their own.[17]
If you have conditions that increase your risk—such as kidney stones, diabetes, or structural abnormalities of your urinary tract—work closely with your healthcare provider to manage these underlying problems. People who have had repeated kidney infections may need additional evaluation to identify and correct any underlying issues that make infections more likely.[14]
How Pyelonephritis Affects Your Body
Understanding what happens in your body during a kidney infection helps explain why the condition can be serious and why prompt treatment matters so much.
When bacteria reach your kidneys, they trigger an inflammatory response. Inflammation (your body’s protective response involving increased blood flow, immune cells, and fluid) is your immune system’s way of fighting the infection, but it also causes the kidneys to swell. This swelling contributes to the pain you feel in your flank and back.[2][6]
The inflammation affects how your kidneys function. Normally, your kidneys filter waste products and excess water from your blood to create urine. During a kidney infection, the inflammatory response causes the kidneys to produce more urine than usual. This increased urine production, combined with fever and sometimes vomiting, can lead to dehydration. The combination of inflammation and dehydration can result in scarring of kidney tissue.[6]
Even a single episode of acute pyelonephritis can cause permanent kidney scarring. This scarring reduces the kidney’s ability to function properly over time. Scar tissue doesn’t work like normal kidney tissue—it can’t filter blood or produce urine effectively. If enough scarring accumulates, it can contribute to high blood pressure and reduced kidney function. This is why treating kidney infections quickly is so crucial, especially in children, whose developing kidneys are particularly vulnerable to scarring.[6]
In the renal pelvis—the funnel-shaped area where urine collects before draining into the ureter—bacteria multiply and produce pus. Pus (a thick fluid containing dead white blood cells, bacteria, and tissue debris) appears in the urine as cloudiness and is why doctors test for white blood cells when diagnosing kidney infections.[2]
The infection can affect one kidney or both kidneys simultaneously. When both kidneys are infected, the impact on kidney function is more severe, and the person typically feels sicker. In some cases, the infection can spread beyond the kidneys. Bacteria from the infected kidney can enter the bloodstream, a condition called bacteremia. When bacteria in the bloodstream trigger a severe immune response throughout the body, it leads to sepsis (a life-threatening condition where the body’s response to infection causes widespread inflammation), which is a medical emergency.[2]
In pregnant women, kidney infections pose additional risks. The infection can increase the likelihood of premature delivery and potentially affect the baby’s health. The physical changes of pregnancy—including pressure on the bladder and changes in hormone levels that affect the urinary tract—make pregnant women more susceptible to infections that can progress quickly.[4]
When a kidney infection occurs in someone who has a blockage in their urinary tract, a particularly dangerous situation can develop. If infected urine becomes trapped behind a blockage—such as a kidney stone—pressure builds up in the kidney. This condition requires urgent surgical drainage because antibiotics alone cannot effectively treat an infection when urine cannot drain properly.[2]
Repeated kidney infections can lead to chronic changes in the kidneys. The ongoing cycle of infection and inflammation creates progressively more scarring, eventually leading to chronic kidney disease if not properly managed. This is why people who have had multiple kidney infections need careful follow-up and evaluation to identify any correctable problems that might be making them prone to repeated infections.[2]
