I give it to mine. You can simply pick it up at a casual encounter with another dog. As it states below, it is airborne, therefore, you can expose your dog by simply going to the vet's office. All you need is another dog with the disease.
Bordetella is the most common cause of tracheobronchitis (kennel cough) in dogs. It is a bacterial illness that is most common among dogs that congregate at things like shows, kennels or other places frequented by large numbers of dogs at once. It does not appear that this bacterin gives a full year of immunity in many instances. For dogs that are often exposed to situations in which the infection is likely probably should be vaccinated twice a year. There are intra-nasal as well as subcutaneous bacterins available for this disease. The intra-nasal bacterin confers immunity more quickly but the injectable version may last longer.
http://www.vetinfo.com/dencyclopedia/debort.html
What is Canine Cough?
Canine infectious tracheobronchitis (Canine cough) is one of the most prevalent infectious diseases in dogs. Fortunately, the majority of cases are not serious resolving on their own in 1 to 2 weeks. The main cause of canine cough is the
airborne bacteria, Bordetella bronchiseptica. A dog with Canine Cough will develop a coarse, dry, hacking cough about five to ten days after being infected. It sounds as if the dog needs to "clear it's throat" and the cough will be triggered by any extra activity, drinking water, exposed to change of temperature or exercise. Many dogs that acquire Canine Cough will cough every few minutes, all day long. They will wretch and sometimes vomit a white foamy looking matter. Their general state of health and alertness are usually not affected, they usually have no rise in temperature, and do not loose their appetite.
Even in the most hygienic, well ventilated, spacious dog facilities, the possibility of a dog acquiring Canine Cough exists. Canine Cough can be acquired from your neighbor's dog, from a Champion show dog at a dog show, from the animal hospital where your dog just came in for treatment of a cut paw, from the sidewalk where an infected dog walked earlier. Try not to blame anyone or any place if your dog develops Canine Cough. There may have been an infected dog, unknown to anyone, that acted as a source for other dogs. The signs of Canine Cough usually will last from 7 to 21 days and can be very annoying for the dog and the dog's owners. If you suspect your dog has canine cough, isolate your dog and visit your veterinarian for medical advice. Do not take your dog to public places in order to prevent the spread of infection. If your dog has recently been to daycare, training, boarding or any other place where your dog was exposed to other dogs, notify the appropriate individuals to advise that your dog is coughing.
http://www.petopia.ca/faq.asp