Most urinary tract infections (UTIs) in children are caused by bacteria that enter the urethra and travel up the urinary tract. Bacteria that normally live in the large intestine and are present in stool (feces) are the most common cause of infection. Occasionally bacteria traveling through the blood or lymphatic system to the urinary tract are the cause of kidney or bladder infections.

During potty training, children may not properly wipe their bottoms after a bowel movement. Bacteria may get into the urethra and cause a UTI. Also, constipation or not completely emptying the bladder can cause bacteria to build up in urine.

Problems with the structure or function of the urinary tract commonly contribute to UTIs in infants and young children. Problems that limit the body's ability to eliminate urine completely include:

• A structural problem of the bladder that allows urine to flow backward to the kidneys (vesicoureteral reflux).
• An obstruction, such as a kidney stone, that blocks, slows, or disrupts the normal flow of urine through the urinary tract (obstructions in the urinary tract).
• Other abnormalities of the urinary tract.