What Is a Urinary Tract Infection?A bacterial urinary tract infection (UTI) is the most common kind of infection affecting the urinary tract. Urine, or pee, is the fluid that is filtered out of the bloodstream by the kidneys. Urine contains salts and waste products, but it doesn't normally contain bacteria. When bacteria get into the bladder or kidney and multiply in the urine, a UTI can result.
There are three main types of UTI. Bacteria that infect only the urethra (the short tube that delivers urine from the bladder to the outside of the body) cause urethritis (pronounced: yur-ih-threye-tis). Bacteria can also cause a bladder infection, which is called cystitis (pronounced: sis-tie-tis). Another, more serious, kind of UTI is infection of the kidney itself, known as pyelonephritis (pronounced: pie-low-nih-fry-tis). With this type of UTI, a person often has back pain, high fever, and vomiting.