Kidney Infection
A kidney infection (pyelonephritis) is a painful and unpleasant illness caused by bacteria travelling from your bladder into one or both of your kidneys. It is more serious than cystitis, a common infection of the bladder that makes urinating painful.
If treated promptly, a kidney infection doesn't cause serious harm, but will make you feel very unwell. If a kidney infection isn't treated, it can get worse and cause permanent kidney damage.
Symptoms of a kidney infection often come on within a few hours. You can feel feverish, shivery, sick and have a pain in your back or side.