Bacterial cystitis is inflammation of the bladder, usually caused by a bacterial pathogen. In the majority of cases of cystitis, a single bacterial pathogen ascends from the urethra into the bladder and causes infection. Cystitis can also be caused by viral infections and as a consequence of chemotherapeutics.

Typical symptoms include dysuria, urinary frequency, and urinary urgency. Some patients may also have suprapubic pain, urinary incontinence, or hematuria. In children old enough to report symptoms, symptoms are similar to adults. In infants, symptoms may be nonspecific and include failure to thrive or vomiting.

Pyelonephritis refers to infection of the kidney and is a different clinical entity. If the patient has flank pain or fever, the physician should consider the possibility of pyelonephritis.

The phrase "complicated urinary tract infection" is used to indicate that host factors exist that may make the urinary tract infection (UTI) difficult to treat. Some of these risk factors include significant health care exposure (increases risk of colonization with resistant organisms), male sex, pregnancy, immunosuppression, recent antibiotic use, indwelling catheter, the presence of foreign body, or an abnormality of the urinary tract.

Of note, pyuria or even a positive urine culture should not be interpreted as evidence of an infection in the absence of compatible urinary symptoms. Asymptomatic bacteriuria is common in certain patient populations (including the elderly) and only requires treatment in very specific situations (patients undergoing certain urologic procedures).