Urinary tract infections involve the parts of the body — the kidneys, ureters, bladder and urethra — that produce urine and carry it out of the body. Urinary tract infections often are classified into two types based on their location in the urinary tract:
Lower tract infections — These include cystitis (bladder infection) and urethritis (infection of the urethra). Lower urinary tract infections commonly are caused by intestinal bacteria, which enter and contaminate the urinary tract from below, usually by spreading from the skin to the urethra and then to the bladder. Urethritis also may be caused by microorganisms that are transmitted through sexual contact, including gonorrhea and Chlamydia. Another form of male urinary infection is prostatitis which is an inflammation of the prostate.
Upper tract infections — These involve the ureters and kidneys and include pyelonephritis (kidney infection). Upper tract infections often occur because bacteria have traveled upward in the urinary tract from the bladder to the kidney or because bacteria carried in the bloodstream have collected in the kidney.
Most cases of urinary tract infections occur in women. Of those that occur in men, relatively few affect younger men. In men older than 50, the prostate gland (a gland near the bottom of the bladder, close to the urethra) can enlarge and block the flow of urine from the bladder. This condition is known as benign prostatic hyperplasia or BPH. This condition can prevent the bladder from emptying completely, which increases the likelihood that bacteria will grow and trigger an infection. Cystitis is more common in men who practice anal intercourse and in those who are not circumcised. Other factors that increase the risk of urinary infections include an obstruction, such as that caused by a partial blockage of the urethra known as a stricture, and non-natural substances, such as rubber catheter tubes (as may be inserted to relieve a blockage in the urethra).