Normally, urine is sterile. It is usually free of bacteria, viruses, and fungi but does contain fluids, salts, and waste products. An infection occurs when tiny organisms, usually bacteria from the digestive tract, cling to the opening of the urethra and begin to multiply. The urethra is the tube that carries urine from the bladder to outside the body. Most infections arise from one type of bacteria, Escherichia coli (E. coli), which normally lives in the colon. In many cases, bacteria first travel to the urethra. When bacteria multiply, an infection can occur. An infection limited to the urethra is called urethritis. If bacteria move to the bladder and multiply, a bladder infection, called cystitis, results. If the infection is not treated promptly, bacteria may then travel further up the ureters to multiply and infect the kidneys. A kidney infection is called pyelonephritis. Microorganisms called Chlamydia and Mycoplasma may also cause UTIs in both men and women, but these infections tend to remain limited to the urethra and reproductive system. Unlike E. coli, Chlamydia and Mycoplasma may be sexually transmitted, and infections require treatment of both partners. The urinary system is structured in a way that helps ward off infection. The ureters and bladder normally prevent urine from backing up toward the kidneys, and the flow of urine from the bladder helps wash bacteria out of the body. In men, the prostate gland produces secretions that slow bacterial growth. In both sexes, immune defenses also prevent infection. But despite these safeguards, infections still occur. http://kidney.niddk.nih.gov/kudiseases/pubs/utiadult/ So what they are saying is don't hold it! I know now, it shall be useful to me.

Women get them easier than men and its basically when bacteria gets into your urethra. Thats why doctors recommend that people wear cotton underwear, wear nothing too tight and never wait for too long when you need to go to the toilet!!

To add to what the others of said (Super panda got it totally right with the science stuff), this advice is for women, as it usually effects them: Bacteria can be 'massaged' into the ureathra during sex, always try to go within 15 minutes of intercourse. Don't wear synthetic underwear or tight trousers as this can cause a hot sweaty enviroment, perfect for bacteria to breed. Wipe from front to back, to avoid spreading bacteria from the anus (e-coli can often be a cause) Go to the toilet as soon as you need to go. Drink lots of water, to keep your system flushing. Cranberry juice is also believed to help prevent it as it stops the bacteria from clinging on to the ureathra.