They are very common. In fact according to the National Kidney and Urologic Diseases Information Clearinghouse, respiratory infections are the only infections that are more common than UTI's. UTI's send people to the doctor over nine and a half million times a year.
Women are most at risk.
In men the prostate is also considered part of the urinary tract and can be infected. Causes Urine is normally sterile. That means it doesn't contain living organisms such as bacteria, viruses or fungi. Infections occur when something gets into the system and starts to multiply. You've heard of it as E. coli.