Pyelonephritis is actually a part of urinary tract infections, which are divided into cystitis and acute pyelonephritis. The main symptoms of cystitis are frequent urination, urgency, and painful urination, among other urinary irritative symptoms, and these do not involve fever. However, if cystitis is not treated in time, it can worsen into acute pyelonephritis, which then presents with symptoms such as fever and back pain. The fever with acute pyelonephritis can be quite high, often exceeding 38 or 39 degrees Celsius, and might last for two to three days. At this point, treatment requires anti-infective medications, which should be continued for two weeks without any interruption. If the treatment duration is insufficient, pyelonephritis may recur. It is important to drink plenty of water, urinate frequently, and avoid holding in urine. Drinking lots of water can help flush out bacteria from the urine early on, promoting a quicker recovery from the illness.