What Are the Symptoms of a Kidney Infection?

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You feel genuinely sick — feverish, aching in your back, maybe nauseated — and you are trying to work out whether this is a bladder infection you can outlast or something that needs a doctor today. That question has a real answer, and this page gives it plainly: a kidney infection — doctors call it pyelonephritis — is a urinary infection that has climbed from the bladder up into one or both kidneys, and its symptoms are the signs of that climb.

This guide lists those symptoms one by one, draws the line between a bladder infection and a kidney infection, and names the combinations that mean getting care now rather than waiting. It is general education only — no page can diagnose you, and this illness is not one to sort out alone. If the picture below sounds like you, the sorting should happen in a clinic or an emergency room, today.

The Short Answer: The Symptoms, in One List

For the reader who is scanning this while feeling awful, here is the whole list up top. A kidney infection typically brings several of these together:

• Fever — often 101°F or higher — with chills, sometimes hard shaking chills
• Pain in the back or side, just under the ribs — the flank — usually on one side
• Nausea and vomiting
• Burning when you urinate, and needing to go often or urgently
• Urine that is cloudy, bloody, dark, or strong-smelling, or painful to pass
• Feeling very unwell all over — exhausted, achy, washed out in a way a bladder infection alone rarely causes

And one more that belongs on this list even though it is not a urinary symptom at all: in older adults, a kidney infection may announce itself mainly as sudden confusion or unusual weakness — sometimes with few or none of the symptoms above. There is a full section on that below, because families miss it most.

If This Sounds Like You Right Now: Get Care Today

A kidney infection is a same-day illness. It is not a wait-and-see infection, and cranberry juice and patience are not a plan for it. If you recognize the picture on this page — above all, fever together with pain in your back or side under the ribs — call your doctor, an urgent care clinic, or a hospital today and say exactly that: fever and flank pain. Those four words move you to the front of the line, because clinicians hear them the way this page means them.

Go to emergency care, or call 911, if any of the following are true:

• You have fever with back or flank pain and cannot reach a doctor quickly
• You are vomiting, or cannot keep fluids or medicine down
• You have shaking chills you cannot control
• You feel confused, hard to wake, or not yourself
• You are passing very little urine, or none
• You feel faint, or your pain is severe and getting worse

Do not plan to ride this out at home and see how the night goes. Untreated, a kidney infection can spread from the kidney into the bloodstream — a dangerous, whole-body illness called sepsis — and that spread can happen faster than people expect. Getting seen the same day is not an overreaction to this infection. It is the correct reaction.

The Backbone of It: An Infection That Climbed

Almost every kidney infection starts lower down. Bacteria — most often ordinary gut bacteria that have wandered where they do not belong — settle in the bladder and cause the familiar bladder infection: burning, urgency, trips to the bathroom every twenty minutes. Miserable, but contained, and relatively low. The urinary tract, though, is a connected road: bladder, then the two tubes called ureters, running up to the kidneys. A kidney infection is what happens when the bacteria travel that road and reach the kidney itself.

That climb is why the symptom picture changes. A bladder infection stays low and local. An infection in the kidney involves a large, blood-rich organ doing essential work, and the body responds like it means it — fever, chills, deep aching pain, nausea, a whole-body sense of being ill. Fever plus flank pain is the line between the two illnesses. Burning alone is usually the bladder talking. Fever and pain under the ribs mean the infection has very likely climbed, and climbed infections are treated with more urgency, stronger follow-through, and no home experiments. Our companion guide on how to tell a kidney stone, a UTI, and a kidney infection apart lays the three side by side, since they share a neighborhood and are easy to confuse.

Usually only one kidney is infected, which is why the pain is usually one-sided. Both can be involved, and in people whose kidneys are already struggling, that matters more — a point this page returns to below.

Each Symptom, in Plain Words

Fever and chills. The fever of a kidney infection is typically a real one — often 101°F or higher — and it often arrives with chills: feeling cold to the bone under blankets, teeth chattering, sometimes shaking hard enough to rattle the bed. Shaking chills in particular are a sign clinicians take seriously, because they can mean bacteria are reaching the bloodstream. A low, mild temperature with bladder burning is one picture; a spiking fever with shaking is another one entirely.

The flank pain. The pain sits in the back or the side, just under the ribs — higher and deeper than ordinary low-back pain, and usually on one side. It tends to be a steady, deep ache rather than a sharp spasm, and the area is often tender: pressing on it, or a clinician tapping it gently with the side of a fist, hurts. People describe it as feeling bruised from the inside. Because it lives in the back, it is often mistaken at first for a pulled muscle — the giveaway is the company it keeps. A pulled muscle does not come with fever, chills, and nausea.

Nausea and vomiting. The kidneys sit close to the digestive system and share its wiring, and a serious infection there commonly turns the stomach: waves of nausea, loss of appetite, vomiting. This symptom matters twice over — it is miserable in itself, and it can make the treatment harder, because a person who cannot keep fluids or an antibiotic pill down may need care in a hospital, where both can be given through a vein.

The bladder symptoms underneath. Burning when you urinate, needing to go often, needing to go now, and passing only small amounts each time are the bladder's complaints, and they often ride along when the infection has climbed — the bladder, after all, is usually where it started. But they do not have to be present. A kidney infection can arrive with surprisingly little burning, especially if the bladder chapter was brief or quiet. Do not let the absence of burning talk you out of the fever and the flank pain.

The urine itself. Infected urine is often cloudy, sometimes with visible blood — pink, red, or tea-colored — and it may smell strong or foul. Passing it may hurt. Any of these changes alongside fever strengthens the case that the urinary tract is where the trouble lives.

The whole-body feeling. People with kidney infections consistently describe being sick in a way that goes beyond the urinary tract: bone-tired, achy all over, no appetite, no energy, sometimes a racing pulse. That whole-body quality is the immune system fighting an infection in a major organ, and it is one of the clearest differences between this illness and a simple bladder infection.

The Symptom Older Adults May Show Instead: Sudden Confusion

This section matters enough to stand on its own, because it is the one most often missed — by patients and by families alike. In older adults, the body's alarm system changes. A kidney infection in a person in her seventies or eighties may not produce a dramatic fever, and the urinary symptoms may be faint or absent entirely. Instead, the infection announces itself in the brain and the body as a whole:

• Sudden confusion — not making sense, not recognizing the day or the room, agitation or unusual quiet
• Unusual weakness or sleepiness — struggling to get up, sleeping far more than normal
• Loss of appetite and not drinking, sometimes with a fall
• A family member's plain report: she's just not herself today

Some older adults never run a fever with a serious infection at all. So the rule for families is this: new confusion in an older person is a medical symptom, not a mood — and when it arrives with any urinary change, any fever at all, or a recent bladder infection, a kidney infection belongs high on the list of suspects. Mention the confusion first when you call. It changes how fast clinicians move.

Why It Matters More on Already-Struggling Kidneys

A kidney infection is a serious illness for anyone. On this site's ground, it is more serious still, because the people most likely to get one are the people with the least kidney to spare. The risk runs higher, and the illness runs harder, in people with:

• Chronic kidney disease — kidneys already filtering below full strength
• Diabetes — which both feeds urinary infections and blunts the body's defense against them
• Kidney stones or any blockage of urine flow — trapped urine behind a blockage is an infection waiting for a spark, and an infected, blocked kidney is an emergency of its own
• A weakened immune system, from illness or from medicines that suppress immunity
• A urinary catheter, or a bladder that does not empty fully

An infection is not a passing insult to a kidney, either. A severe kidney infection can injure the organ it infects and leave scarring behind — damage that kidneys already struggling can ill afford — and, as noted above, it can seed the bloodstream. There is a heart connection here as well, a brief one: a serious infection forces the heart to work far harder, faster, against a body in crisis, and a heart that is already weak can struggle under that load. It is one more way the same cluster of conditions — diabetes, high blood pressure, kidney disease, heart disease — makes each illness heavier than it would be alone. How these infections start, who gets them, and what genuinely helps prevent them is the subject of our companion article, Can I Get a Kidney Infection?

What the Doctor Will Do

The visit is usually straightforward. The doctor will ask about your symptoms — when the fever started, where the pain sits, what your urine looks like — and examine you, including that gentle tap over the flank that people with this infection reliably wince at. Then come tests:

• A urine test, and usually a urine culture — a sample checked for signs of infection and grown in the lab to identify the exact bacteria, so the antibiotic can be matched to it
• Blood tests, which can show how hard the body is fighting and how the kidneys are coping
• Sometimes imaging — an ultrasound or scan — if a stone or blockage is suspected, if the infection is severe, or if it is not improving as expected

Kidney infections are treated with antibiotics — that is not optional or a matter of style; the infection is bacterial, deep in an organ, and antibiotics are what clear it. Many people are treated at home with pills. Some are treated in the hospital, at least at first — people who are vomiting, dehydrated, very ill, pregnant, older with other conditions, or whose kidneys or immune systems are already compromised — where antibiotics and fluids can go directly into a vein. Which setting is right depends on the person in front of the doctor, and needing the hospital is not a failure; it is the treatment matching the illness.

Recovery — and the Signs That Mean Go Back

On the right antibiotic, most people begin to turn the corner within about two days: the fever eases, the pain loosens its grip, the stomach settles. Full recovery — energy back, appetite back — takes longer, often a week or more, and tiredness that lingers a while is normal after an illness like this. Take the antibiotic exactly as prescribed and finish the course, even once you feel well; stopping early is how an infection gets a second act.

Call your doctor or go back in — the same day — if:

• Your fever has not begun to improve after about 48 hours on the antibiotic
• You are vomiting again, or cannot keep the medicine down
• The flank pain is getting worse, not better
• You are passing much less urine, feel faint, or become confused

A fever that will not yield can mean the antibiotic does not match the bacteria, or that a stone or blockage is sheltering the infection — both fixable, but only if the care team knows. Going back is not complaining. It is information the treatment needs.

Why This Research Matters

Infections that climb are part of how kidneys are lost — one fever at a time, scar by scar, in organs that often had damage already. The Cardio Renal Foundation supports research toward a solution beyond dialysis — work aimed at the burdens failing kidneys leave behind, so that the road this infection travels has a different ending than it does today. That is a direction, not a promise: nothing in a laboratory changes what a person with fever and flank pain should do, which is get care today. But it is aimed squarely at the stakes this page describes.

At the Cardio Renal Foundation (Cardio Renal Foundation Inc, EIN 33-4359765), we believe plain-language education like this is a genuine first line of defense, and support from readers like you helps us keep publishing it for the patients and families who need it.

This article is informational only and is not meant to replace the advice of your qualified healthcare provider. Nothing here diagnoses any condition or recommends any treatment for you personally. Never start, stop, or change a prescribed medicine without speaking to the clinician who prescribed it. Research supported by this Foundation is investigational only. As with any issues related to your health, check with your doctor.