Most of the time we have no idea why people get urinary tract infections. They are commoner in girls than boys, probably because they have shorter pee tubes (urethra) so that nasties like bacteria have a shorter way to climb up into your bladder if you are a girl. It is called 'urinary' infection because it is an infection of your pee tract which can include your kidneys, the tubes which connect your kidneys to your bladder, your bladder itself (in which case it also called cysitits) or your pee tube from your bladder to the outside world (urethra). Some people get recurrent urinary tract infections because they have funny kinks in their pee tubes, because of stones or because they are diabetic. Sometimes it can be related to having sex.
How do you know that you have got it?
It may hurt to pee; you may pee more often; you may get a fever; you may get blood in your pee; you may get a pain in your side over where your kidneys are.
What should you do about it?
Drink lots of fluid to help flush out the bacteria which are causing the infection; go and see your doctor and get your urine tested - for the infection and if necessary (your GP will tell you the results) get an antibiotic from your doctor.