Dog biting is always a serious issue, but it doesn't always mean that your dog is acting out of simple aggression. Sometimes this behavior has a medical cause. Dog biting can be a result of pain or illness, so rule out those potential causes before you seek a behavioral solution.
How Can You Tell If It's an Illness?
Learn your dog's body language. Pay attention to the way your dog interacts with other dogs and people. While there can be a first time for everything, dogs don't typically act out in unpredictable ways without a good reason. First, try to figure out what triggers the biting; is it protecting food or toys, being approached rapidly by a strange person or dog or any other outward sign of provocation? If your dog has never responded negatively to any of these stimuli, or if you don't see any sign of provocation when biting behavior occurs, it's time to consider medical problems.
What Is the Dog Doing?
Are there any identifiable circumstances surrounding dog biting incidents? Did someone attempt to touch or pick up the dog? If your dog has never responded negatively to touch and suddenly becomes grouchy, or if your dog bites a beloved and trusted family member, it's a good sign of a medical issue.
Take your dog to the vet, and describe the behavior that occurs when the dog tries to bite. Your vet may muzzle the dog for safety reasons. Pain from injured bones, arthritis or internal injuries could all cause your dog to react defensively. As much as you can without risking injury, try to identify the part of your dog's body that's causing the pain. This may help your vet with the diagnosis.
Avoid Touching a Sick or Injured Dog
Most dogs don't bite out of malice when they're sick or injured; it's a natural reaction to protect themselves. If you notice a sick or injured dog, don't walk up to it and try to touch it. Even the most well-mannered dog may bite out of illness, so you don't want to put you or the dog in a situation where an attack may occur. If you see a strange dog that is sick or injured, call animal control and let them come and safely take the animal to a medical facility.
If the family pet is sick or injured and you don't want to wait for animal control to help, try using a thick towel to protect you from defensive bites. Gently place the towel over the dogs head while moving him to prevent him from biting, but be careful not to smother or hurt the dog with the towel. Quickly move him into a crate or other container where you can transport him safely, and then remove the towel so he can travel comfortably.
So you approached crate training with patience and persistence and it is still not working for you and your dog. Crate training is not for everyone and is definitely not for every dog. This does not mean that you have a "bad dog" or that there is something wrong with your dog.