By far the most common cause of white blood cells in the urine is due to an infection of the urinary system - usually of the bladder, but may also be of the kidney. Urinary tract infection accounts for pus cells in the urine 98-99% of the time. This is because the body responds to any form of irritation of the tissues by an inflammatory response, which is manifested by an increase in the blood flow to the area, slight swelling of the tissues, and an infiltration of white blood cells. Bacterial infections are the number one cause of inflammation; they can be isolated on urine culture. A urinary infection can be caused specifically by sexually transmitted infections: gonorrhea, chlamydia, and certain viral infections such as: herpes simplex and human papilloma virus (condyloma /warts). Parasites like Schistosoma haematobium, Giardia lamblia, and Entamoeba histolytica may also be seen in the urine and cause pyuria.
Infection of the prostate gland (prostatitis) may also produce pyuria in association with symptoms of urinary hesitancy, urgency, and urine dribbling.
Stones of the kidney, ureter or bladder typically have blood in the urine, but occasionally will have more white blood cells if they are irritating enough to the urinary system.
All tissues heal with an inflammatory response and thus patients with trauma or who have had surgery on the urinary system will have pyuria until they are healed.
A less common cause in otherwise healthy young individuals is tumors of the bladder or kidney.