Pain causes confusion in a dog. Anyone who approaches a sick or injured dog risks being perceived as the source of the pain, and the dog may bite to stop the pain. When a dog is in pain, he doesn't understand where the pain comes from. If you touch him, he may think you are causing the pain and he could bite you to stop the pain.
Make sure to keep your dog's vaccinations up to date. It is also important to closely monitor their overall health. Irritability often goes hand-in-hand with illness. Making sure your dog is healthy will insure that they are less likely to bite.
Dog bite injuries can be the result of a perceived threat to someone or something the dog cares about, such as an offspring, their food or even something as seemingly unimportant as a toy. Dogs can bite when they believe that a family member is under attack. In that case, a dog can act aggressively to protect their master and family members.
In conclusion, it is usually best to train your four-legged friend when he is still a puppy. If that doesn't work you will need the help of a professional, because a dog who bites (no matter the size) is a potential risk for everyone around it. In nearly all cases of adult dogs that bite, the trouble could have been corrected if the dog was given proper training when it was a puppy.