Cancer is responsible for as many as half of the canine and feline deaths in the United States and is the leading cause of death in dogs older than two years. When a beloved dog is diagnosed with cancer, it’s natural to feel confused, frightened and even responsible. Was it something you did? Most likely, the answer is no. Scientists have spent decades studying this confounding and insidious disease, and to date there is no definitive conclusion as to why some humans and animals get cancer. However, evidence continues to mount linking it to environmental toxins, where everything from common household cleaners and pesticides to flea and tick preventives and vaccines have been shown to cause carcinogenic mutations to the cells.