Your urinary system includes your kidneys, ureters, bladder and urethra. All play a role in removing waste from your body. Your kidneys — a pair of bean-shaped organs located toward the back of your upper abdomen — filter waste from your blood and regulate the concentrations of many substances. Tubes called ureters carry urine from your kidneys to the bladder, where it's stored until it exits your body through the urethra.
UTIs typically occur when bacteria outside the body enter the urinary tract through the urethra and begin to multiply. Most cases of cystitis are caused by a type of Escherichia coli (E. coli) bacteria.
Bacterial bladder infections may occur in women as a result of sexual intercourse. But even sexually inactive girls and women are susceptible to lower urinary tract infections because the female genital area often harbors bacteria that can cause cystitis.
Courtesy: Mayo Clinic