Pyelonephritis, also known as upper urinary tract infection, is an inflammatory response caused by pathogens; 95% of pyelonephritis is caused by bacterial infections. Most cases involve bacteria from the urethral opening ascending into the urinary system to reach the kidneys, where they proliferate and cause inflammatory symptoms. Normally, there are bacteria present at the human urethral opening. Under normal immune function or absence of factors like significant urinary obstruction, it is unlikely for an upward bacterial infection to occur. However, under certain conditions, this type of infection may happen, prompting an inflammatory response from the pathogens.