Infection from above can occur in the urinary tract, and they can also occur from below as well. As far as above, the bacteria goes to the kidneys from the bloodstream and heads downward. When it is below, the bacteria enters through the urethra and continues upward.
You will usually see the infections from above in newborns with sepsis or general infections. If there is a significant amount of bacteria in the bloodstream, there is a chance that some of them will make it through the bodies filters to the urine. This is more likely to happen if the filters are not mature or there is a large amount of bacteria.
In adults and children that are older, you will usually see infections that start from below. When you are dealing with babies that are wearing diapers, you will have a situation in which the bacteria can enter through the urethra and travel upward. The longer the dirty diaper sits there, the more likely it is to happen. It is more likely to happen in baby girls though than baby boys, because the distance that the bacteria has to travel is shorter in girls, and the head of the penis is less likely to be sitting in stool when boys have a dirty diaper.
A lot of girls are first taught to wipe front to back in order to prevent UTI’s, because when they wipe back to front it is a lot more likely to lead to bacteria getting in the vaginal area. Those women that are sexually active are more likely to have bacteria pushed through the vaginal area and into the urethra.