The urinary tract (which consists of the kidneys, the ureters, the urinary bladder and the urethra) is free from any infection/ organism/bacteria.

On the other hand, the intestinal canal usually contains organisms, like Escherichia coli (E. coli), which passes out in large numbers in the faeces. Under normal circumstances, they do not cause any harm in the intestine, but they are extremely harmful to the urinary tract when they enter the urethral orifice from the anal orifice. This happens especially when no proper washing/ cleaning is done after each defecation, and the area outside the anal orifice remains contaminated with E. coli. It is, therefore, the urinary tract infection, especially ,recurrent, is more common in women since the anal as well as the urethral orifice/ opening is lying close together, and the bacteria can easily enter from the anal to the urethral opening. Moreover, the urethra is very short in women (only an inch long), so infection reaches the urinary bladder in no time. On the other hand, the urethra in males is much longer, and is much farther away from the anus or anal orifice, and thus chances of infection from the anus to the urethra are significantly reduced.

However, infection in the urinary tract may also reach there through the blood, or through lymphatics, and besides E. coli, other bacteria may also damage the urinary tract, especially when the infection is carried by a catheter, directly into the urinary bladder.

As soon as the organisms reach the urinary bladder, they start multiplying in the urine present in it, which happens to be an excellent medium for their growth. Chances of the growth of these organisms become much more in any part of the urinary tract, e.g. the kidneys, the ureters, etc., when there is some obstruction in the urinary passages. For example, a stone, a stricture or some congenital abnormality (like horseshoe kidneys, etc.) which does not allow the free passage of urine from the kidneys to the urinary bladder / urethra, and thus allowing it to stay much longer than required, above the site of the obstruction.

This collection/stagnation of urine facilitates the growth or multiplication of bacteria. If the obstruction is in either part of the ureter, the infection/ growth of the organisms will occur above the obstruction in the ureter and in the kidneys of the side involved. However, if the obstruction is much lower down, say in the urethra, as a result of an enlarged prostate, in males, which is a common occurrence, the growth/infection will be in the urinary bladder, and may, in advanced cases, go higher up, in both the ureters and in both the kidneys as well.

In the urinary bladder, the organisms cause marked inflammation, in which case pus cells are formed, which are passed out in the urine. Once the urinary bladder is infected, and the patient is not careful, and does not take proper antibiotics or takes half-hearted treatment, the infection goes up to the kidneys through the ureters. The infection of the urinary bladder is called cystitis, and of the kidneys, pyelonephritis.

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