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A urinary tract infection (UTI) is a condition where one or more structures in the urinary tract become infected after bacteria overcome the structures' strong natural defenses. Almost 95% of cases of UTIs are caused by bacteria that typically multiply at the opening of the urethra and travel up to the bladder. Much less often, bacteria spread to the kidney from the bloodstream. UTIs are classified as primary or recurrent, depending on whether they are the first infection or whether they are repeat events. Cystitis is the most common urinary tract infection and is sometimes referred to as acute uncomplicated UTI. It occurs in the lower urinary tract.