Parvovirus (PAR-vo-vie-rus) is a fairly new disease, first appearing in 1977, but it's deadly. It usually occurs in younger dogs, since the virus thrives in rapidly-dividing cells. There are two forms: one that centers in the digestive tract and another in the heart. Symptoms include lethargy, lack of appetite, vomiting and diarrhea. Once the symptoms occur, there is no treatment and the disease is almost always fatal. Fortunately, the condition is preventable. Virtually all standard vaccination now include parvo, and are highly effective. Puppies should be vaccinated when they're six to eight weeks old, with follow-ups every two to three weeks until the puppy is twenty weeks old. A parvo booster with the dog's annual checkup should keep your pet free from this disease.