A kidney infection (pyelonephritis) typically causes fever, back or side pain, and urinary symptoms like painful urination, urgency, or frequency. You may also experience nausea, vomiting, chills, or cloudy/bloody urine. Unlike a simple urinary tract infection, kidney infections are more serious and often come with systemic symptoms—particularly high fever (often above 100.4°F) and flank pain on the affected side. Some people have no back pain but feel generally ill. Symptoms usually develop over hours to a few days. If you suspect a kidney infection, seek medical attention promptly because these infections require antibiotic treatment and can become severe if untreated. A doctor confirms diagnosis through urine cultures, blood tests, and sometimes imaging. Risk factors include untreated UTIs, urinary blockages, pregnancy, or a weakened immune system. Men with kidney infection symptoms should especially seek care, as kidney infections are less common in men and may indicate a more serious underlying problem. Don't self-treat with over-the-counter remedies alone—kidney infections need professional evaluation to prevent complications like sepsis or permanent kidney damage.