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Urinary tract infection - Causes of a urinary tract infections

NHS Choices Medical Reference

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Most urinary tract infections (UTIs) are caused by bacteria that live in the digestive system.

If these bacteria get into the urethra (the tube where urine comes out) they can cause infection.

It's thought that the bacteria can spread to the urethra via the anus. If toilet paper touches your anus and then your genitals when you go to the loo, the bacteria can then multiply and move through your urinary tract, causing infection of your:

  • urethra (urethritis): the urethra is the tube that carries urine from the bladder to the hole where it leaves the body (in men this is at the tip of the penis, in women it's between the vagina and the clitoris)
  • bladder (cystitis): the bladder is where urine is stored until we urinate
  • ureters (ureteritis): the ureters are tubes that run from the bladder to the kidneys
  • kidneys (pyelonephritis): the kidneys are two bean-shaped organs that remove waste materials from your blood and convert it to urine

Who's at risk of getting a UTI?

Women are more likely than men to have a UTI. This is because in women the urethra is closer to the anus than it is in men, which makes it easier for bacteria to get from the anus to the urethra. In women the urethra is also much shorter than it is in men, making it easier for bacteria to reach the bladder.

You are also more likely to develop a UTI if:

  • You have a condition that obstructs or blocks your urinary tract, such as kidney stones.
  • You have a condition that prevents you from fully emptying your bladder; if urine stays in the bladder for too long it's easier for bacteria to multiply.
  • You have a weakened immune system - due to, for example, chemotherapy or HIV.
  • You have a urinary catheter, which is a tube that a doctor or nurse inserts into your bladder to drain away the urine.

Further risk factors in women

If you're a woman, you are also more likely to get a UTI if: 

  • You're sexually active - having sex can irritate the urethra, allowing bacteria to travel through it more easily and into your bladder.
  • You use a diaphragm for contraception, as a diaphragm can put pressure on the bladder and prevent it from emptying properly.
  • You use condoms that are coated in spermicide, as spermicide can irritate the vagina making it more vulnerable to infection.

Further risk factors in men

If you're a man, you are also more likely to get a UTI if:

  • you have an enlarged prostate gland, as this can put pressure on the bladder and urethra, preventing the bladder from emptying properly

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Medical Review: April 07, 2012
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