How do you treat a dog with a kennel cough?

ALWAYS. It's scientific name is tracheobronchitis. It is similar to bronchitis in human beings in that it involves either a viral or bacterial infection with expectorating cough, can progress to pneumonia, is quickly and easily contagious, and is treated with antibiotics. In that, there is good news, for while kennel cough is easily caught, it is also fairly easy to ameliorate. Get thee dog to a veterinarian.