A urinary tract infection is an infection involving part or all of the urinary tract. The effects of the infection depend on the interaction between the bacterium and the host’s defence mechanisms.

If the brunt of the infection is situated in the bladder, the symptoms tend to be of a local nature and the disease is called cystitis, or a lower urinary tract infection. Infection involving mainly the kidney is called pyelonephritis, or an upper urinary tract infection. In pyelonephritis the symptoms tend to be of a more systemic nature, such as fever, chills and a fast heart rate.

The division between upper and lower urinary tract infection is somewhat arbitrary, since the infection enters the system by an ascending route via the urethra (the tube connecting the bladder to the outside) in both cases. The bacterium E coli is responsible for 80% of urinary tract infections.

TB of the urinary tract, kidney abscess and infections of the male genital organs (prostate, testicles and epididymis) are dealt with in related topics (see below). Infections of the female genital tract are not regarded as urinary tract infections.