I have a 4 year old terrier mix. We have had him since he was 12 weeks and he is now about 8 pounds. He has always been a nervous dog but recently has gotten aggressive and bitten two of our neighbors. One was a child that was in our home playing with my son. When she came up the stairs the dog ran over barking and bit her leg. The next time we were sitting outside and our neighbor walked over to say hi. The dog got very aggressive and ran towards her growling and barking and bit her twice. I don't know what to do. I don't know why he has started biting after this amount of time. I am at a loss. Please help.