What causes a urine infection?

Urinary infection: Urine and the urinary tract, is ordinarily sterile. "Something" happens, that allows bacteria to take hold. This is what's called a UTI or urinary tract infection. Sometimes it's a blockage of some type (think kidney stone) or other structural issue. Sometimes it's exposure or insufficient hygiene. E. coli is the germ most classically associated with UTIs. If you have such an infection, see a doc.