What is a one-sentence summary of the following article?
Kennel cough is extremely contagious. If your dog has been playing with other dogs in a park or has been staying in a kennel, there is a possibility it has been exposed. A dog infected with kennel cough may suddenly develop a cough, which can vary in severity from a quiet, persistent "huff" to a harsh hacking, choking cough. The latter is commonly mistaken for the dog having something stuck in his throat. If possible, open his mouth to check for a stick or bone that is stuck. An alternative means of determining if a dog has something stuck in its throat is to offer the dog a treat to eat. A dog with something stuck in his throat will not be able to eat the treat, so if he eats and swallows it without difficulty, it is unlikely it has a foreign object in its throat. Just as humans get a sore throat with flu, so do dogs with kennel cough. This can lead to throat clearing, or gagging and retching episodes. For some dogs, this is so severe that they retch up saliva or froth. A dog that is vomiting because of nausea (rather than due to excessive coughing) will bring up yellow bile or food from the stomach. This is a likely a sign of some other problem. Some dogs with kennel cough show no signs of being ill, apart from an unpleasant cough. Others can be sluggish, lacking in energy and losing their appetites. It is always a good idea to get a dog that is coughing seen by your vet, but it is essential if the dog suddenly lacks energy or doesn't eat for 24 hours.

Summary:
Understand the risk factors. Listen for coughing. Watch for gagging. Monitor your dog's energy.