If your dog is behaving uncharacteristically lethargic, anxious, or displays other symptoms of illness (sneezing, coughing, vomiting, diarrhea, etc.), you should ALWAYS consult a veterinarian. The following links provide general information on illnesses that affect dogs, how to identify signs of illness, and how to understand a diagnosis given by a vet:
In addition to watching for signs of illness, Dogs XL recommends yearly well pet checkups for your dog to detect serious illness before the dogs displays any symptoms. Dogs can't tell us when they are in pain, so it is extra important to provide routine veterinary care.