A bacterial infection of the urinary tract, most often caused by the bacterium E. coli. They are more common in women than in men. They usually affect the bladder (cystitis), but can also affect the kidneys (pyelonephritis). Symptoms of a bladder infection usually begin with frequent urination, an urgent need to urinate, and pain with urination (dysuria). Symptoms may also include fever, cloudy urine, foul smelling urine, or blood in the urine (hematuria). If the infection affects the kidneys, symptoms will usually also include high fever, chills, back pain on the side of the affected kidney, nausea, and vomiting.