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Your kidneys are the organs in your body that make urine. You have two of them, each one about the size of a fist. They sit near the middle of your back, just below your ribs. They filter water and waste products out of your blood to make urine.

Urine drains from your kidneys down tubes called ureters. You have two ureters, one from each kidney. Both ureters empty into your bladder. Urine is stored in your bladder until you go to the bathroom. When you urinate, urine passes out of your body through a tube called the urethra.

The part of your body that makes and gets rid of urine is called your urinary tract. Your urinary tract includes your kidneys, ureters, bladder and urethra.

You can get an infection in your urinary tract when tiny organisms, usually bacteria, get into your urethra and start to multiply.¹

An infection that stays in your urethra is called urethritis. If it spreads to your bladder, it's a bladder infection, also called cystitis. Doctors sometimes call infections in your bladder or urethra lower urinary tract infections.

An infection in the lower part of your urinary tract may travel up your ureters to infect one or both kidneys. The medical name for a kidney infection is pyelonephritis. Doctors sometimes call kidney infections upper urinary tract infections.

If a doctor just says "urinary tract infection" or "UTI", they probably mean urethritis or a bladder infection. To read more, see our information on Bladder infection.

There are two main types of kidney infection:^{2 3 4}

• Uncomplicated kidney infections. This is when a person with a normal urinary tract and a healthy immune system gets infected with a common type of bacteria. This type of kidney infection is most common in women aged 18 to 40.
• Complicated kidney infections. This is when the infection is more serious for some reason. It might be caused by bacteria that aren't as easy to kill with drugs. Or you could have another medical condition that makes your kidney infection worse, or means your body can't fight it off as well as it should.

You may be more likely to get a complicated kidney infection if you:

• Are over 60
• Have a blockage or another problem with your urinary tract
• Have kidneys that don't work properly
• Have a kidney stone
• Need a catheter (a tube to drain urine from your bladder)
• Have a problem that affects your immune system. Your immune system is made up of all the parts of your body that help fight disease.
• Have recently had your bladder and urethra examined with an instrument called a cystoscope
• Are pregnant
• Get infected by less common bacteria that are difficult to treat with antibiotics.

Men are more likely than women to get complicated kidney infections. That's partly because they have prostate glands, which can swell and block the flow of urine.

If you have a complicated kidney infection, you're likely to feel more sick than you would with an uncomplicated infection.⁴ You may need to be treated in the hospital.

Most kidney infections are acute. This means they come on suddenly and go away after treatment. But some people get a chronic kidney infection. Chronic is a word doctors use to describe a condition you have for a long time. It doesn't mean that a condition is any more serious. People usually only get a chronic kidney infection if they also have another medical problem with their kidneys.⁵

The bacteria that cause kidney infections nearly always reach your kidneys by traveling up from your urethra and your bladder. Sometimes, the infection reaches your kidney through your bloodstream. But this is rare in healthy people.³

More than 8 in 10 uncomplicated kidney infections are caused by bacteria called Escherichia coli, or E. coli for short.²

E. coli also causes more than half of all complicated kidney infections. People with this type of infection often have strains of E. coli that are more difficult to treat with antibiotics.⁶

Anyone can get a kidney infection, but there may be things about you and your life that make it more likely. These are called risk factors. For women, the risk factors for kidney infections are the same as the risk factors for bladder infections. Risk factors for women include:⁷

• Having sex (including oral sex or anal sex) often. We don't know exactly why sex increases your risk of infection, but it probably makes it more likely that bacteria can get into your body from the area around your genitals.⁸
• Using a spermicide for contraception. Spermicide kills some of the protective bacteria that live in your vagina.⁸
• Having a new sex partner in the last year
• Having had a urinary tract infection, such as a bladder infection, in the last year
• Having a mother with a history of urinary infections
• Having diabetes
• Leaking urine, for example when you cough or sneeze. To read more, see our information on Stress incontinence.

This information is for educational use only, and is not a substitute for prompt professional medical advice. Readers should always consult a physician or other professional for advice and treatment. ©BMJ Publishing Group Limited 2008. All rights reserved.