I was hoping to get some responses back from my request for more information, but don’t want you to keep waiting too long either.
Usually in cases where a dog is aggressive, it is often just a case of training. However,the first thing that should be done is to rule out a medical cause for the sudden aggression. You can read about these here:
If there is no medical cause for the aggression, then it is strictly behavioral. Dogs are aggressive toward people for a variety of reasons. It might be that they are fearful of people and thus are aggressive before the person can be. In other cases, a dog is aggressive in order to dominate the people. Other causes could be that the dog feels they are the alpha member of the pack and as the alpha member they must protect the pack (you) from threats (people).
In addition, owners sometimes make the situation even worse by tensing up and worrying about what will happen. The dog senses the owner worry and feels that he is justified in his aggressive stance because you are obviously worried about the people. They don't know you are worried about them attacking, they just feel that you are worried and assume it is the people. This doesn’t seem to be the case with your dog though.
For a dog like this, total control is necessary. This means not only physical control but on a mental level, you must be the leader or boss. You will need to obedience train him.. If you can, I would do group classes and let the trainer know of the problem your dog has.. Before you can get into classes, I am including a great site for teaching owners to teach their dogs. Be sure and click on the link to the left on obedience. and links on subsequent pages leading to detailed instructions.
Training works best if you train at least 30 minutes a day (two 15 minute sessions). I would start making your dog work via the Nothing in life is free program (NILF). It is outlined below.
This obedience work establishes you as the boss and as the boss it is your job to protect the dog and not the dogs job to protect you. Now the dog will still protect you if something should happen because you will be protecting yourself and as part of the pack, he will follow your lead and protect as well. But if you are calm around someone, he will see that you are ok and there is no need to help. This is why being the leader works well.
You will also want to keep a leash on him at all times initially to grab if he should disobey. I think you will be pleasantly surprised at how well your dog does with training. Dogs like knowing what is expected of them and they love the little paper thin slices of hotdogs that I use for treats while training.
It will be helpful if you can find someone to help you once you have your dog listening to commands consistently. What you will do is have your dog on the leash. You will have your helper off in the distance. Your helper will gradually move closer to you preferably walking past your position in the distance. As long as your dog ignores them, you can give your dog praise and a treat. The second you see her fixate on the person or show any other sign of aggression (hair standing up, etc.) give your dog a correction by giving a short tug and a firm low toned "NO". It shouldn't take your dog long to realize you will not tolerate the aggression and that if se ignores the person, he gets treats. Once this happens you can repeat the training moving the person closer until he is no longer trying to lunge at people. You will need to practice this when you and your dog are walking as well.
You can do a similar exercise while having people enter your yard. It should be ok for the dog to bark so you know someone is there, but once you give the quiet command, they should stop and then you can have him sit and stay. As crazy as this sounds you may wish to teach the pup the speak command and then the quiet command. It seems easier to teach the quiet command after the dog has learned the speak command. The following site explains teaching speak and quiet commands.
In addition, if the situation is not improving using the techniques on the previous website, you may have to consult a professional behaviorist. You can usually find a behaviorist by asking your Vet for a recommendation or you may be able to find one using the following site.
Unfortunately, at this point, I would seriously consider having a behaviorist anyway available to evaluate the dog and determine the aggression level. In the meantime, institute the techniques I have recommended.