I'm so sorry you got this dog from a Pet store. These dogs come from puppy mills and are never healthy.
PUPPYMILLS.COM
The dogs don't have to drink out of the same water dish to catch kennel cough. It's an airborn virus. Other pets can catch it simply being in the same area even if they are kept seperate.
Kennel Cough - Dog Kennel Cough Information - Kennel Cough In Dogs Treatments