Most urinary tract infections (UTIs) in children are caused by bacteria that normally live in the digestive system getting into the urethra (the tube that carries urine out of the body).

In young children, bacteria can enter the urethra when they wipe their bottom after going to the toilet, and soiled toilet paper comes into contact with their genitals.

This is more of a problem for girls than boys, because girls' bottoms are much nearer the urethra.

Babies who soil their nappies can also sometimes get small particles of stool into their urethra, particularly if they squirm a lot when being changed.

Increased risk

There is usually no obvious reason why some children develop UTIs and others do not.

However, some children may be more vulnerable to UTIs due to a problem with bladder emptying. Normally, we empty our bladder completely when going to the toilet. If urine is left in the bladder, it can allow bacteria to grow and an infection to develop.

Problems that can affect bladder emptying include:

• constipation – this can sometimes cause part of the large intestine to swell, which can put pressure on the bladder and prevent it from emptying normally
• dysfunctional elimination syndrome – a relatively common childhood condition where a child "holds on" to their urine, even though they have the urge to urinate
• vesicoureteral reflux – an uncommon condition where urine leaks back up from the bladder into the ureters (the tubes connecting the kidneys and bladder) and kidneys. This occurs due to a problem with the valves in the ureters, where they enter the bladder