By definition, parasites are beings that can only exist through the help, willing or not, of another living being. In dogs, this category includes internal and external pests such as worms, ticks, and fleas. These creatures must be associated with a dog or other animal for at least part of their lifetime. Not only do they drain nutrients from dogs, they cause a wide variety of health problems and may even transmit diseases. Your goal as a responsible dog owner should be to keep your dog as parasite-free as possible for a long, healthy life.