A kidney infection typically feels like a deep, steady pain in your lower back or side, often accompanied by fever, chills, and nausea that come on suddenly. Unlike a regular urinary tract infection, which stays in the bladder and causes mainly burning during urination, a kidney infection makes your whole body feel sick. Most people describe feeling hit with flu-like misery on top of a persistent ache that doesn’t go away no matter how you move or position yourself.
Where the Pain Shows Up
The hallmark of a kidney infection is pain in the flank, the area on either side of your spine just below the rib cage and above the hips. It usually affects one side, though both kidneys can be involved. The pain tends to sit deep in one spot rather than radiating down the legs like a pinched nerve would. Some people also feel it spread into the lower abdomen or inner thighs.
One reliable way to tell kidney pain from a pulled muscle or spinal issue: kidney pain does not change with movement. Muscle and back pain typically flares when you twist, bend, or shift positions, and it eases when you find a comfortable one. Kidney infection pain stays constant regardless of what you do, and it won’t improve on its own without treatment. During a medical exam, your doctor will thump gently on your back just below the last rib. If that produces a sharp jolt of tenderness, it strongly suggests the kidneys are inflamed.
The Whole-Body Feeling
What separates a kidney infection from a simple bladder infection is how profoundly unwell you feel. A bladder infection is annoying. A kidney infection can put you flat on your back. Common systemic symptoms include:
- Fever and chills: A temperature above 100.4°F (38°C) is characteristic, and shaking chills or rigors are common. Mild or early cases sometimes skip the fever, but most people spike one.
- Nausea and vomiting: Many people feel too sick to eat and may vomit repeatedly.
- Fatigue and malaise: A heavy, drained feeling that goes beyond normal tiredness.
These symptoms often come on fast. Rather than building gradually over days, a kidney infection is more likely to hit suddenly, going from “I feel a little off” to “I feel terrible” within hours.
Changes in Your Urine
Because a kidney infection is essentially a UTI that has traveled upward from the bladder, you may still have lower urinary tract symptoms: frequent urges to pee, burning or stinging during urination, and a feeling that your bladder never fully empties. But the urine itself often changes too.
Infected urine can look cloudy, darker than usual, or even pinkish or reddish if blood is present. The smell is often noticeably different from normal. Bacteria in the urinary tract can produce odors described as rotten eggs or rotting fish. If your urine looks or smells off and you’re also running a fever with back pain, those pieces together point strongly toward a kidney infection rather than a mild bladder issue.
How It Differs From a Bladder Infection
A standard bladder infection (lower UTI) mainly causes local discomfort: burning when you urinate, pelvic pressure, and frequent trips to the bathroom. You generally don’t feel sick beyond that. A kidney infection layers whole-body illness on top of those urinary symptoms. The fever, the shaking chills, the vomiting, and the deep back or side pain are the red flags that infection has reached the kidneys.
Not everyone follows the textbook progression. Some people develop kidney infection symptoms without ever noticing bladder symptoms first. Others have mild bladder symptoms for a few days before the infection climbs and the more intense symptoms appear. Either pattern is common.
Symptoms in Older Adults and Children
Kidney infections don’t always look the same at every age. Older adults frequently present with atypical symptoms. The classic fever may be absent entirely, especially in frail or immunocompromised people. Instead, the first signs might be confusion, general weakness, or a noticeable change in mental sharpness. Because older adults often already have baseline urinary symptoms from other conditions, the usual burning and frequency can be easy to overlook or dismiss.
Young children, especially infants and toddlers, can’t describe what they feel. A kidney infection in a small child might show up as unexplained high fever, irritability, poor feeding, or vomiting. Any urinary infection in a young child is treated seriously because their kidneys are still developing and more vulnerable to lasting damage.
When Symptoms Signal an Emergency
Most kidney infections respond well to antibiotics and resolve without complications. But in rare cases, the infection can enter the bloodstream and trigger a dangerous condition called urosepsis. The warning signs that things have escalated beyond a manageable infection include a very high fever above 103°F (39.4°C), a rapid or pounding heartbeat, difficulty breathing, feeling lightheaded or faint, and confusion or difficulty staying alert.
A weak pulse, inability to catch your breath, or a sudden drop in blood pressure are signs your body is struggling to contain the infection. These symptoms call for emergency care, not a scheduled appointment. Kidney infections that are caught early and treated promptly almost never reach this point, which is why paying attention to the early combination of flank pain, fever, and feeling suddenly ill matters so much.