Male dog aggression

Question
One of my dogs is usually very calm and nice, but one day, suddenly, he
attacked my friend. He jumped onto him and started biting and scratching my
friend. We then locked him into the garage and washed the wounds. This was
the first time he has ever acted like this. Why do you think he did that?

Answer
I have no idea what breed this dog is, how old he is, what his behavior is like around people, what sort of experiences he may have had with this individual that you are unaware of, what this friend was doing, where the friend had recently been, how the friend reacted to this dog in the past; in fact, without psychic powers, I have no way of answering this question.  I would need a great deal more information and details of veterinary history, diet, other physical symptoms, where the dog sleeps, how many other dogs you have...in fact, you need a first person observer who can interview you extensively and diagnose the problem.  Find a credentialed animal behavior expert (Ph.D. or DVM, not a dog trainer) and keep this friend and this dog away from one another in future.