Try to buy dogs from acquaintances and dogs from merchants. After long-distance trafficking, most of them carry infectious diseases and viruses. It is difficult to judge latent infectious diseases simply by visual inspection.
Before buying a dog, you can buy your own vaccine and inject it into the dog owner's home. You are required to pick up the dog after half a month. In case of death, the dog owner can bear part or all of the losses through agreement.
This is my personal experience, which may inspire you.