If you buy a dog from a pet store or commercial breeder, California's so-called Puppy Lemon Law gives you some protections in case the animal becomes ill shortly after purchase.
The law, officially known as the Pet Breeder Warranty Act, applies to cases in which the purchased dog gets sick due to an illness or disease that existed within 15 days of purchase. It also applies if problems arise in the first year after the sale because of a congenital or hereditary condition.
If you want to keep the dog, the law entitles you get your money back from the seller plus up to an additional 50% of the purchase price for veterinary costs.
The law also allows you to return the dog, like a piece of merchandise, and get a refund or a different dog of equivalent value. Still, you'd be entitled to reimbursement of "reasonable" veterinary costs.