Oral-health

The guy who works at my local video shop has terribly rotton teeth- one of his front teeth has completely rotton off. He says he has a disease. His older brother's teeth are slightly greenish near the gums. But his younger sisters' teeth are fine. I just want to know if there really is a tooth decaying disease that isnt caused by a lack of personal cleanliness. I must mention that he lives off sweets and coke- he is always chewing on biltong or eating some kind of sweets whether it be chocolate or toffees or anything. Iv only ever seen him eat proper food once- and that was kfc. He also looks greasy and smells musty. His clothes too. I work in the same center as him and go to the video shop quite often. So I just want to know if there is some genetic tooth decaying disease out there and what are the chances of him having it if his 3 sisters, mother and father don't have that problem.