The Common Causes
Babies are especially vulnerable to UTIs because they're in diapers most of the time, which keeps their genital area moist and warm and allows bacteria to breed. Plus, diapers don't always keep their messes contained, so bacteria from bowel movements can easily get into the genitals and sometimes cause an infection. Because girls have a shorter distance between the end of the urethra and the bladder than boys do, girls seem to have a higher chance of getting an infection this way. And research has shown that uncircumcised boys have about a tenfold greater risk of getting a UTI than circumcised boys do, because bacteria can hide under the foreskin, making it harder to clean.
But your child isn't defenseless. Every time he pees, his urine blasts out any offending bacteria that have gotten into the urethra. Urine is also a mild acid and contains antibacterial proteins that fight invaders. That's why most children get through life without any infections, even though the urethra is near one of the least clean areas of the body.
But the bacteria do fight back and actually sometimes win. Like evil little mountain climbers, some microorganisms have actually evolved to have tiny little hooks that allow them to hang on to the walls of the urethra and avoid being washed away as they climb higher. Many bacteria also have a coating that shields them from a child's immune system.
UTIs can occur for another reason. Just as some adults have acid reflux from stomach contents that bubble up into the esophagus, 43 percent of babies have urinary reflux. In this condition, some urine flows backward from the bladder up into the kidney, instead of down and out through the urethra, and it may carry invading bacteria into the kidney itself. Though urinary reflux may improve as a child grows up, it can cause repeated infections in infancy and early childhood. (More on reflux later.)
The infections are common during the potty-training years. Some headstrong toddlers may hold in their urine for prolonged periods, possibly allowing the bacteria to climb and cause an infection before they're washed away. (Rarely, very frequent UTIs may even signal a problem with the nerves controlling the bladder, which a doctor might suspect if a child also has weakness in his lower body.) A child with severe constipation can have such full intestines that they actually pinch shut the plumbing around the bladder, which makes it harder to pee and causes infections.