The urine travels from the kidneys down two narrow tubes called the ureters. When the bladder empties, urine flows out of the body through the urethra, a tube at the bottom of the bladder. When this happens, the bacteria can infect and inflame the bladder, resulting in swelling and pain in the lower abdomen and side. If the bacteria travel further up through the ureters to the kidneys, a kidney infection can develop. The infection is usually accompanied by pain and fever.
Kidney infections are much more serious than bladder infections.
In some children a urinary tract infection may be a sign of an abnormal urinary tract that may be prone to repeated problems. For this reason, when a child is found to have a urinary infection, additional tests are often recommended. In other cases, children develop urinary tract infections because they are prone to such infections the way, for example, other children are prone to getting coughs, colds, or ear infections. Or a child may happen to get an infection with a type of bacteria that has a special ability to cause urinary tract infections. The lining of the bladder, urethra, ureters, and kidneys become irritated with a urinary tract infection, just like the inside of the nose or throat with a cold.