Kidney infections, also known as pyelonephritis, are usually caused by bacteria that travel up the urinary tract to the kidneys. They can result from untreated urinary tract infections (UTIs) or as a complication of other conditions like kidney stones. Symptoms include fever, pain in the back or sides, painful urination, and nausea. Treatment typically involves antibiotics, and in severe cases, hospitalization may be required. Early treatment is important to prevent kidney damage and other complications. It occurs when bacteria or viruses from the lower urinary tract (such as the bladder) spread to the kidneys. If left untreated, kidney infections can cause permanent kidney damage or lead to life-threatening complications like sepsis.