Coccidia really isnt harmful unless never treated. Almost all puppies will or can get it. But they get treated for it. The reason why a Vet check is done before you get the puppy from the breeder and when you get it . To make sure its healthy. No parasites or is being treated. At that young age one they stop drinking and eating they can goes fast. A good breeder won't let you have the pup at such a young age. Sometimes they won't eat in a new environment. Unless someone is home 24-7 to watch the pup. The old the better. I know the op probably will only get there money back is by getting a new pup. But if I could get my money back I would get it then get a new breeder. JMO.
Coccidia (Coccidiosis) in Dogs: A Cause of Diarrhea