Most likely, your dog contracted kennel cough by coming in contact with another infected dog. Outbreaks of kennel cough result from direct dog-to-dog contact or from airborne contact with the respiratory secretions of a dog with kennel cough. A dog can also get kennel cough by using the same bowl as an infected dog that hasn’t been cleaned properly, or even from a human touching an infected dog and not washing his or her hands before touching another dog. In other words, the illness is spread among dogs the same way the common cold is spread among humans. That’s why most outbreaks occur where large numbers of dogs are kept in close quarters or where they have contact with one another, such as boarding facilities, doggie daycares, dog parks, veterinary hospitals, grooming shops, pet stores, shelters, etc. In fact, research shows that outbreaks of kennel cough may affect more than 50% of dogs in such places.
Dogs that are unvaccinated against the agents that cause kennel cough are of course the most likely to contract the infection. Also, animals who are immunocompromised, or have a weak immune system, are more susceptible to illnesses like kennel cough.