causes of urinary tract infections
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.4  Sexual intercourse may lead to cystitis, but you don't have to be sexually active to develop it.he most common UTIs occur mainly in women and affect the bladder and urethra. 1  Infection of the bladder (cystitis). 2  Infection of the urethra (urethritis). 3  Symptoms. 4  Risk factors.