Pyelonephritis can also be referred to as an upper urinary tract infection. It occurs when bacteria proliferate in parts of the kidney such as the renal pelvis and calyces, leading to inflammation. In most cases, these bacteria enter the urinary system retrogradely from the urethral opening and cause inflammation in areas like the renal pelvis and calyces. Normally, bacteria are already present at the urethral opening, and when the body's immune system is functioning well and the urinary system is unobstructed, it is difficult for these bacteria to enter the urinary system and cause an inflammatory response. However, in patients with weakened immune systems, such as those suffering from a cold, diabetes, or those using steroid medications, bacteria may enter the urinary system and replicate in large numbers. Additionally, if there are obstructive factors in the urinary system, such as prostate enlargement in male patients, or if the patient has urinary system stones or tumors, bacteria are more likely to cause an inflammatory response.