It's true that some people have bad breath. It's quite rare but it does happen. The blame is usually laid at the state of the teeth or gums, but in reality this is rarely the cause.
Us ENT surgeons diagnose tonsillitis in the young by asking the parents "has their breath gone off?". Rotten tonsils colour the breath, for sure. It's quite easy to spot, as often tonsil stones are present. See picture below. Treatment is by removing the superficial part of the tonsil using local anaesthetic spray and the carbon dioxide laser.