Called Pyelonephritis (or simply “Pyelo” for short), it’s caused by E. coli and other bacteria that normally live in the bowel. They can wind up in the nearby urethra, and pass up it into the bladder, and up the ureter to the kidney (see diagram Anatomy of Urinary System). This is more common in women, where the urethral opening is closer to the anus than in men, and who also have shorter urethras than men (in whom the germs likely get urinated out before they have enough time to get to the bladder) (see also diagram Anatomy of the Female Genital System).
Patients with Pyelo have pain & tenderness in the flank or back, & tenderness when tapping on the place where the bottom rib meets the spine. There’s usually nausea or vomiting, and often a fever. Since an infection almost only occurs in just one kidney, we’d expect no pain or tenderness on the other side. Diagnosis is easily made by urinalysis, as long as a clean-catch urine specimen is collected correctly (see link).
Sometimes a patient began first with symptoms of Bladder Infection (aka UTI, cystitis), like painful and frequent urination, but sometimes not (especially not in older or pregnant women, and men). The danger of kidney infection (not simply bladder infection) is that germs can spread to the blood to cause Sepsis (which can be lethal). In pregnancy, there’s risk of miscarriage or premature labor. Treatment with antibiotics is almost always easy. By the way, Pyelo is technically a “UTI” (“urinary tract infection“), but when we use the abbreviation, we’re referring to bladder infections, which are much more common.
Before starting medication, clinicians should obtain a urine specimen for Culture and Sensitivity, to not only identify the bacteria, but also see which antibiotics will and won’t work. People often say, “Antibiotic X works for me,” but that’s not true, because it needs to work on the particular strain of bacteria (which can easily change), not on “you.” Since one dose of any antibiotic can ruin the test, that’s an excellent reason not to begin self-treatment with left-over or borrowed medication.