Feline Pyometra is an infection of the uterus of a cat. It doesn’t come from bacteria crawling in, but rather is due to a hormonal imbalance.
Fluid normally present in the uterus is a great breeding ground for bacteria. Some of the hormones also inhibit white blood cell response and normal contractions which would clear out any infection.
If left untreated, Pyometra is a fatal condition. Cats will become septic, the uterus can rupture and the pus will enter the abdominal cavity. This means a slow and painful death for the cat.
The prevention of Pyometra is early spaying! This is generally a condition of older cats (3+ years) but can occur at any age.