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.