A bladder infection is caused by an infection of the urine. Urine is stored in the bladder until it’s passed out through a single tube called the urethra. Urine is normally sterile, and acts as an antiseptic, washing potentially harmful bacteria out of the body during urination. A urinary tract infection (UTI) occurs when bacteria gets into the urine.
We all have bacteria that normally live harmlessly in the bowel, but if they are left on the skin around the back passage they can sometimes travel to the urethra, especially in women because those two openings are so close together. Most urine infections are due to these bacteria, which thrive in urine and multiply quickly
They mostly lodge in the bladder, when the infection is called cystitis. Read more at Cystitis.