Acute pyelonephritis is a bacterial kidney infection that can cause sudden, severe illness with fever and back pain, requiring prompt medical attention to prevent lasting damage to these vital organs.

Understanding How Common Kidney Infections Are

Acute pyelonephritis affects a significant number of people each year, though it remains less common than infections of the lower urinary tract. In the United States, approximately 1 in 2,000 people develop a kidney infection annually, which translates to roughly 250,000 office visits and 200,000 hospital admissions each year.^[1][3] These numbers reflect the serious nature of the condition and why healthcare systems must be prepared to recognize and treat it effectively.

The disease does not affect everyone equally. Young women between the ages of 15 and 29 experience the highest rates of acute pyelonephritis, making this group particularly vulnerable.^[3][8] After this age group, infants and older adults face increased risk. Annual rates in the general population show about 15 to 17 cases per 10,000 females compared to only 3 to 4 cases per 10,000 males, demonstrating a clear difference between the sexes.^[2] Men generally have relatively low risk of developing acute pyelonephritis unless they are older than 65 years of age.^[7] This pattern exists across different populations and remains consistent year after year.

The difference in rates between women and men relates to anatomical factors. Women have a shorter urethra (the tube through which urine leaves the body), which makes it easier for bacteria to travel upward from outside the body to the bladder and eventually to the kidneys.^[1] This structural difference explains why women account for the vast majority of uncomplicated kidney infection cases. However, when men do develop pyelonephritis, it often signals an underlying problem with the urinary tract that needs investigation.

What Causes Kidney Infections

Bacterial infections cause the overwhelming majority of acute pyelonephritis cases. The most common culprit is a bacterium called Escherichia coli, better known as E. coli, which accounts for approximately 75 to 95 percent of uncomplicated kidney infections.^[3][13] This bacterium normally lives in the digestive tract without causing problems, but when it enters the urinary system, it can trigger serious infections. Other bacteria that can cause kidney infections include Proteus mirabilis, Enterobacter, Staphylococcus, Klebsiella species, and various other members of the Enterobacteriaceae family.^[1][7]

The path these bacteria take to reach the kidneys follows a predictable route in most cases. Bacteria present in the gastrointestinal tract come into contact with the opening of the urethra. From there, they move into the bladder and multiply. If the infection is not stopped at the bladder level, the bacteria can travel upward through tubes called ureters that connect the bladder to the kidneys.^[7][9] Once bacteria reach the kidney tissue, they trigger inflammation and the body’s immune response, creating the symptoms of acute pyelonephritis. This ascending pattern of infection explains why many people with kidney infections first experience symptoms of a bladder infection before their condition worsens.

In rare situations, bacteria can reach the kidneys through the bloodstream rather than by climbing up from the bladder. This happens when bacteria from an infection elsewhere in the body travel through the blood and settle in the kidney tissue.^[7] This route of infection is less common and typically occurs in people with weakened immune systems or other serious health conditions. Understanding how bacteria reach the kidneys helps explain why prompt treatment of bladder infections can prevent more serious kidney involvement.

Risk Factors That Increase Your Chances

Several factors can increase a person’s likelihood of developing acute pyelonephritis. Having a prior urinary tract infection is one of the strongest risk factors, as bacteria from an untreated or incompletely treated bladder infection can move upward to the kidneys.^[4] This connection explains why addressing lower urinary tract symptoms promptly matters so much for preventing more serious complications. Many women who initially present with bladder infection symptoms actually have kidney involvement that becomes apparent when short-course treatment fails.^[2]

Physical blockages in the urinary system create conditions where bacteria can multiply and spread more easily. Anything that prevents complete emptying of urine from the urinary tract allows bacteria to grow and back up into the kidneys. These blockages include kidney stones, an enlarged prostate in men, and uterine prolapse in women.^[1] Pressure on the bladder during pregnancy also increases risk by making it harder to fully empty the bladder. A condition called vesicoureteral reflux, where urine flows backward from the bladder toward the kidneys instead of moving in the normal direction, can carry bacteria upward and lead to kidney infections.^[1][7]

Certain medical conditions put people at higher risk for developing kidney infections. Diabetes stands out as an important risk factor because high blood sugar can affect immune function and make it harder for the body to fight infections.^[4][7] People with HIV, those taking medications that suppress the immune system, and individuals undergoing chemotherapy all face increased vulnerability to infections including pyelonephritis. Foreign objects in the urinary tract, such as catheters or kidney stones, provide surfaces where bacteria can attach and multiply.^[4]

Behavioral and lifestyle factors also play a role. Frequent sexual intercourse can introduce bacteria to the area around the urethra, increasing the chance of infection, especially when combined with the use of spermicide-containing contraceptives.^[7][4] Spermicides destroy the natural protective bacteria that should be present in the vaginal area, allowing harmful intestinal bacteria to colonize instead. Having new sexual partners can also increase risk of urinary tract infections. Women whose mothers have a history of urinary tract infections may be more vulnerable to kidney infections themselves, suggesting a possible hereditary component.^[4]

Recognizing the Symptoms

The symptoms of acute pyelonephritis often appear suddenly and make people feel quite ill. Fever is one of the hallmark signs, typically reaching 38°C (100.4°F) or higher, and may be accompanied by chills and shaking.^[4][7] However, fever might be absent early in the illness or in certain groups such as older adults. Back pain, especially in the area just below the rib cage and above the waist on either side of the spine, represents another distinctive feature. This flank pain is nearly universal in kidney infections, and its absence should make doctors consider other diagnoses.^[2][4]

Many people with pyelonephritis also experience symptoms related to urination. Pain or burning when urinating, needing to urinate frequently, and feeling an urgent need to urinate are common complaints.^[1][9] The urine itself may look cloudy or bloody and often has an unusual or unpleasant smell. Some people notice that they are producing less urine than normal. These urinary symptoms overlap with those of bladder infections, but kidney infections typically cause people to feel much sicker overall.

Gastrointestinal symptoms frequently accompany kidney infections. Nausea and vomiting are common and can make it difficult to keep down food, fluids, or medications.^[4][7] Abdominal pain may be present alongside the characteristic back pain. The person may feel generally weak, achy, or as if they have the flu. This combination of fever, back pain, and feeling systemically unwell helps distinguish a kidney infection from a simple bladder infection, where people usually feel less sick and do not run high fevers.

The presentation can be quite different in very young children and elderly adults. Newborns with kidney infections may refuse to feed, vomit, or simply have a fever without other obvious symptoms.^[6] Children under two years old may have fever without reliably showing other signs, making diagnosis more challenging. In older adults beyond 65 years, the typical symptoms might be absent or subtle. Instead, they may present with confusion, jumbled speech, or hallucinations as their main symptoms.^[7] This atypical presentation means kidney infection might not initially be suspected in these age groups, potentially delaying diagnosis and treatment.

Preventing Kidney Infections

Preventing acute pyelonephritis centers largely on preventing urinary tract infections from developing or stopping them before they spread to the kidneys. Drinking plenty of water throughout the day helps by encouraging frequent urination, which flushes bacteria out of the urinary system before they can multiply and cause infection.^[19] However, people with kidney, heart, or liver disease who need to limit fluid intake should talk with their doctor before increasing how much they drink. The simple act of urinating regularly, rather than holding urine for long periods, also helps prevent bacteria from growing in the bladder.

Prompt treatment of bladder infections can prevent them from progressing to kidney infections. Anyone experiencing symptoms of a bladder infection, such as burning during urination or needing to urinate frequently, should contact a healthcare provider quickly so the problem can be treated before it spreads upward to the kidneys.^[19] This is particularly important because a significant number of kidney infections develop from untreated or inadequately treated bladder infections. Finishing the full course of antibiotics prescribed for any urinary tract infection, even if symptoms improve before the medication is gone, helps ensure the infection is completely cleared.

For women, several specific preventive measures can help reduce risk. Urinating soon after sexual intercourse helps flush out any bacteria that may have been introduced near the urethra.^[19] Wiping from front to back after using the toilet prevents bacteria from the anal area from spreading toward the urethra. Changing sanitary pads frequently during menstruation and avoiding douches and deodorant-containing feminine hygiene products also supports urinary tract health. These products can disrupt the natural balance of protective bacteria in the vaginal area. For men, keeping the tip of the penis clean helps prevent bacterial growth that could lead to infection.

Addressing underlying health conditions that increase risk is another important preventive strategy. People with diabetes should work to keep blood sugar levels well controlled, as this helps the immune system function properly and resist infections.^[4] Those with structural abnormalities of the urinary tract may need surgical correction to prevent recurrent infections. Anyone with recurrent urinary tract infections should work with their healthcare provider to identify contributing factors and develop a personalized prevention plan.

How Kidney Infections Affect the Body

When bacteria invade the kidney tissue, they trigger a cascade of changes in how the kidneys normally function. The infection causes inflammation of the renal parenchyma (the functional tissue of the kidney), the calices (cup-shaped structures that collect urine), and the renal pelvis (the funnel-shaped area where urine collects before moving to the bladder).^[4] This inflammatory response represents the body’s attempt to fight off the infection, but it also causes tissue swelling and damage. The kidneys may become enlarged as immune cells rush to the area and inflammation develops.

The inflammation and infection interfere with the kidney’s normal job of filtering waste and excess fluid from the blood. As the kidneys respond to the bacterial invasion, they may produce more urine than usual, which can lead to dehydration if the person is also vomiting and unable to drink enough fluids.^[6] At the same time, the inflamed kidney tissue may be less efficient at concentrating urine and reabsorbing water, further contributing to fluid loss. This combination of increased urine production and decreased fluid intake due to nausea explains why people with kidney infections can quickly become dehydrated.

In the blood, markers of inflammation and infection rise. White blood cell counts typically increase as the immune system mobilizes to fight the bacteria. Inflammatory proteins such as C-reactive protein and erythrocyte sedimentation rate become elevated, reflecting the body’s active response to infection.^[4] The kidneys themselves may temporarily lose some of their filtering capacity, which can be detected through blood tests showing elevated waste products. In severe cases, acute kidney injury can develop, where the kidneys suddenly lose much of their ability to filter blood and maintain fluid balance.

The physical and biochemical changes caused by just one episode of acute pyelonephritis can potentially result in permanent scarring of kidney tissue if the infection is not treated promptly and effectively.^[6] This scarring can lead to long-term problems including high blood pressure and reduced kidney function. Repeated kidney infections over time cause cumulative damage and increase the risk of chronic kidney disease. In particularly severe cases, especially in people with weakened immune systems, the infection can spread to the bloodstream, causing sepsis, a life-threatening condition where the body’s response to infection causes widespread inflammation and organ dysfunction.^[1]