What a great post.  It is true what you say. Periodontal disease and other viruses and bacterial infections can lodge and stay within a dog or cat's mouth and create havoc.  I rescued a desert cat who lived near the sand dunes in the Middle East and brought her back home with me. Several years later she developed a disease where all her teeth had to be removed when she was 17years old.  I had all her teeth removed as per the vet who specialized in periodontal disease in animals, he is actually the periodontal vet for the Phoenix Zoo. She really became so ill and was near death. After the procedure and a short bout of medication, all was well.  She is now 22 and litterally runs to get her food when called.  Love the series you are writing.