Answer: dog... cat... pyometra... spay
     Seen more often in dogs than cats, pyometra means pus in the uterus.  Pus (white blood cells that accumulate in defense of infection) buildup can escape through the uterine tract and is seen as a discharge from the vulva and often collects on the fur under the tail and back legs.  In some cases no discharge is present... called a closed pyometra.  Various factors play into the development of pyometra such as irregular heat cycles, bladder or vaginal infections, hormone imbalance, and false pregnancy episodes.
     The uterine cavity becomes infiltrated with pus and mucous from the infection and distends, stretching and thinning the uterine wall.  Bacteria and toxins enter the bloodstream and cause havoc with the kidneys, liver and other organs and may lead to endotoxic shock and sudden death. Veterinarians  treat pyometra as an emergency surgical case and are reluctant to simply "try some antibiotics".   Pyometra can be fatal!

Doctor's Notes
     There are a number of health enhancing effects on dogs and cats from being spayed or neutered.  In female dogs and cats any chance of pyometra is eliminated by being spayed. Spaying means the ovaries and uterus are removed and is referred to as an "ovariohysterectomy".

False Pregnancy...
     This condition occurs when the dog or cat feels, looks, and acts like she is pregnant and even produces milk in the mammary glands, and yet no fetuses are present.  It is more common in dogs than cats.