The Cause

Litter box: The ripeness of Snowball's litter box depends on the litter as well as on her diet, allergies, or infections, and whether the box is cleaned daily.
Fur: Dirty coats breed odor-causing bacteria. "If we went months between baths, we'd stink," says Robert Krapfl, a veterinarian in Omaha, Nebraska.
Accidents: Fluids spread as they seep into carpet padding and create nasty smells.

The Cure

Litter box: Clay-based litter that clumps when wet, such as Scoop Away Plus Crystals ($10 to $13 for a 25-pound box, at supermarkets) absorbs odors.
Fur: Brushing and bathing can prevent the buildup of bacteria. Ask your vet about shampoos.
Accidents: Spray Nature's Miracle Stain & Odor Remover (about $8 at pet stores) on an area wider than the spot (and under it, if possible).