A kidney infection typically causes fever, pain in your back or side, and painful urination. These symptoms often develop quickly, sometimes within a day, and tend to feel noticeably worse than a simple bladder infection. Knowing what to look for matters because kidney infections can become serious if left untreated.
The Most Common Symptoms
Kidney infections produce a combination of urinary symptoms and whole-body signs that set them apart from less serious infections. The hallmark symptoms include:
- Fever and chills, often with temperatures reaching 100.4°F (38°C) or higher
- Pain in your back, side, or groin, typically on one side
- Frequent or painful urination
- Nausea or vomiting
- Cloudy, dark, bloody, or foul-smelling urine
The side or back pain is one of the most telling signs. It usually sits just below the ribs on the affected side, in the area where the kidney sits. During a physical exam, a doctor will tap on this spot. Sharp tenderness there is a strong indicator that the infection has reached the kidney rather than staying in the bladder.
How Kidney Infection Feels Different From a Bladder Infection
Most kidney infections start as bladder infections (lower urinary tract infections) that travel upward. A bladder infection on its own causes local symptoms: burning when you urinate, feeling like you need to go constantly even when your bladder is empty, pressure in your lower abdomen, and changes to your urine’s appearance or smell.
A kidney infection layers systemic symptoms on top of those. Fever, chills, nausea, vomiting, and pain radiating into your back or side are the key additions. If you had a few days of typical bladder infection symptoms and then develop a fever with flank pain, that progression strongly suggests the infection has moved to one or both kidneys. The shift usually feels obvious. You go from uncomfortable to genuinely sick.
Symptoms in Children
Children show kidney infection symptoms differently depending on their age. Kids 2 and older may describe the same back or side pain, fever, and painful urination that adults experience. They might also start wetting the bed again after being fully toilet trained, or they may complain of strong urges to urinate with very little coming out.
Babies and toddlers under 2 are harder to read. A high fever may be the only visible sign. They can also become unusually fussy or irritable, refuse to eat, gain weight poorly, or develop vomiting and diarrhea. Because young children can’t describe where it hurts, a persistent unexplained fever in a baby always warrants checking for a urinary tract infection.
Symptoms in Older Adults
Older adults, especially those over 65, frequently present with atypical symptoms that are easy to miss. The classic burning during urination may not appear at all. Instead, sudden confusion, increased agitation, new incontinence, loss of appetite, or unexplained falls can be the primary signs that an infection is present.
Any type of physical stress, including infection, can cause confusion in older adults. For someone already living with Alzheimer’s disease or dementia, a urinary tract or kidney infection can make cognitive symptoms temporarily and noticeably worse. Caregivers who notice a sudden behavioral shift, even a subtle one, should consider infection as a possible cause.
Symptoms During Pregnancy
Pregnant women develop kidney infections more easily because hormonal and anatomical changes slow the flow of urine, giving bacteria more time to multiply. The symptoms are the same: fever of 100.4°F or higher, flank pain, nausea, and vomiting alongside urinary symptoms.
What makes pregnancy different is the stakes. Kidney infections during pregnancy are associated with higher rates of preterm delivery and low birth weight. In severe cases, the infection can progress to sepsis or respiratory complications. This is why routine urine screening during prenatal visits exists: catching and treating even symptom-free bacterial growth in the urine helps prevent kidney infections from developing in the first place.
Warning Signs the Infection Is Spreading
A kidney infection that enters the bloodstream can cause a dangerous condition called urosepsis. This is rare when infections are treated promptly, but it’s important to recognize the warning signs:
- Rapid heart rate or heart palpitations
- Breathing faster than normal (more than 22 breaths per minute) or difficulty breathing
- A drop in blood pressure, which can cause dizziness, lightheadedness, or feeling faint
These symptoms represent a medical emergency. If you’ve been dealing with kidney infection symptoms and notice your heart racing, you feel short of breath, or you become confused and lightheaded, you need emergency care immediately.
What Happens After Diagnosis
Doctors typically confirm a kidney infection through a urine test that checks for white blood cells (your body’s infection fighters) and bacteria. The location of your pain, your fever, and tenderness when your back is examined all factor into the diagnosis as well.
Treatment is antibiotics. Most people start feeling better within a few days of beginning their course, though the full prescription needs to be completed to clear the infection entirely. Mild to moderate cases are usually treated at home. More severe infections, particularly those with high fevers, persistent vomiting, or signs that the infection may be spreading, may require a stay in the hospital for closer monitoring and stronger treatment.
If you’ve had one kidney infection, it’s worth paying attention to bladder infection symptoms in the future and treating them early. Most kidney infections are preventable when lower urinary tract infections are caught before they have a chance to climb upward.