The severity of pyelonephritis depends on many factors such as the type of pathogen, patient’s age, gender, immune status, and other co-morbidities. It may result in bloodstream infection (known as sepsis) if left untreated, which has a very high mortality rate. Treatment of pyelonephritis could be complicated in elderly individuals, pregnant patients, patients with immunosuppression, patients with a current kidney stone, and patients with a recent history of hospitalization and antibiotic treatment and it could be fatal in these groups of patients.