Pyelonephritis
Acute pyelonephritis refers to an infection of the kidneys. It is often sudden and may cause severe damage to the kidneys. Bacterial infection of the kidneys causes them to swell, and it is accompanied by severe pain in the flanks.
When bacteria cause repeated or persistent attacks, the condition is termed as chronic pyelonephritis. It is although rare, but happens more often in pregnant women, children, and people with urinary obstructions.