The most common UTIs occur mainly in women and affect the bladder and urethra. 1  Infection of the bladder (cystitis). 2  This type of UTI is usually caused by Escherichia coli (E. coli), a type of bacteria commonly found in the gastrointestinal (GI) tract. 3  However, sometimes other bacteria are responsible. This type of UTI can occur when GI bacteria spread from the anus to the urethra. 2  Also, because the female urethra is close to the vagina, sexually transmitted infections, such as herpes, gonorrhea, chlamydia and mycoplasma, can cause urethritis.