Bladder infections, also known as urinary tract infections (UTIs), are caused by bacteria that enter the urinary tract through the urethra and begin to multiply. The most common bacteria that cause bladder infections are Escherichia coli (E. coli), which are normally found in the bowel. Other types of bacteria can also cause bladder infections. Women are at greater risk of developing bladder infections than men because their urethra is shorter and closer to the anus, making it easier for bacteria to enter the urinary tract. Sexual activity can also lead to bladder infections, but it is not necessary to be sexually active to develop one. Some noninfectious factors can also cause bladder inflammation, such as interstitial cystitis.

In summary, bladder infections are caused by bacteria, usually E. coli, that enter the urinary tract through the urethra and begin to multiply. Women are at greater risk of developing bladder infections than men because of their anatomy.