A kidney infection is a painful, unpleasant illness that usually happens when bacteria travels up from your bladder into one or both of your kidneys.
It's different from - and more serious than - cystitis, which is a common infection of the bladder that makes urinating painful.
Kidney infection, medically known as pyelonephritis, doesn't usually pose a serious threat to your health if treated promptly, but it can make you feel very unwell. And, if a kidney infection isn't treated, it can get worse and cause permanent kidney damage.
Often the symptoms come on quickly, within a few hours, and they can make you feel feverish, shivery, sick and with a pain in your back or side.
When to see your GP
You should see your GP quickly if you have symptoms of a kidney infection.
Most kidney infections need prompt treatment with antibiotics. That's to stop the infection from damaging the kidneys or spreading to the bloodstream. You may need painkillers too.
If you're especially vulnerable to the effects of an infection, for example if you have a pre-existing health condition or you're pregnant, you may be admitted to hospital as a precaution and treated with antibiotics that are given by intravenous drip into a vein.
Read more about treating a kidney infection.
What causes a kidney infection?
The kidneys are two bean-shaped organs on either side of the body, just underneath your ribcage. Their main role is to filter out waste products from blood. These waste products, along with excess fluid, are then converted into urine and passed out of the body.
A kidney infection usually happens when bacteria, often a type called E. coli, accidentally gets into the urethra (the tube through which urine passes out of the body) from the anus and then travels up through the bladder into one of the kidneys.
In most cases of kidney infection, only one kidney is affected.
Read more about the causes of a kidney infection.
Who's at risk?
Kidney infections aren't that common. It's estimated that 1 in every 830 people will develop a kidney infection in any given year.
They can happen at any age, but are much more common in women. In fact, women are six times more likely to get a kidney infection than men. This is because a woman's urethra is shorter than a man's, which makes it easier for bacteria to reach their kidneys.
Younger women are most at risk because they tend to be sexually active, and having frequent sex increases the chances of getting a kidney infection. Kidney infections are also slightly more common during pregnancy.
Younger children are also vulnerable to developing kidney infections because their small urinary tract makes it easier for bacteria to reach their kidneys. It is estimated that 1 in 20 cases of high temperature in children is actually a result of a kidney infection.
There are steps you can take to reduce your risk of kidney infection. Find out more about how to prevent kidney infection.