Pyelonephritis, also known as an upper urinary tract infection, indeed tends to cause fever in patients. Generally, this type of fever can rise sharply in a short period of time, potentially exceeding 39°C. The duration of the fever mainly depends on the patient's own resistance and the effectiveness of the medication. If sensitive antibiotics are used and the patient has strong immune capabilities, the bacteria can be killed or mostly eliminated in a short period of time. In such cases, the patient's high fever may gradually subside within three to five days. However, if the medication is ineffective, the fever may persist for an extended period, possibly lasting up to half a month or even a month. (The use of medications should be carried out under the guidance of a professional doctor.)