The key to getting a healthy yorkie is to find a reputable breeder with healthy dogs. A good breeder will know their lines, have them tested for genetic issues, and will only breed genetically healthy dogs. If you buy from a petstore, puppymill, or backyard breeder - there is a lot more chance of you getting a dog that will have health problems down the road. Luxating patellas and Liver Shunt are genetic diseases that are common in yorkies.
So far as just general health - my Lacy has been a very healthy dog. She's three years old and the only problems she has had have been an eye infection and a bladder infection. And of course the occasionally just "not feeling well"..diarrhea and vomiting...but that never lasts long.
Good Luck in your yorkie search and welcome to YT! It's great that you're doing research before you bring a new baby home
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~Magnifique Yorkies~
Purchasing from backyard breeders, pet shops, and puppymills perpetuates the suffering of other dogs.
Educate yourself and buy from reputable breeders or rescue.