Causes of kidney infection
A kidney infection usually happens when bacteria – often a type called E. coli – get into the tube that carries urine out of your body (urethra). The bacteria travel up to your bladder, causing cystitis, and then up into your kidneys.
E. coli bacteria normally live in your bowel, where they cause no harm. They can be transferred from your bottom to your genitals during sex or if you're not careful when wiping your bottom after going to the loo.
A kidney infection can sometimes develop without a bladder infection – for example, if you have a problem with your kidney, such as kidney stones, or if you have diabetes or a weakened immune system.
source: NHS choice
