There are many diseases that can affect very young dogs. This could be an infectious disease like parvo virus, intestinal parasites, traumatic injury, or be a congenital problem. Puppies of this age do not have a fully developed immune system and are very fragile. This dog needs to be examined by a veterinarian as soon as possible. Sick puppies get dehydrated, hypothermic, and hypoglycemic very quickly and fade away fast.