My dad chews tobacco. What can happen to him from it? And is it dangerous to the rest of us?

My dad has been addicted to chewing tobacco for as long as I can remember; I'm 15 years old now. I'm beginning to worry about him, he's been chewing for so long but he seems okay, can something still happen to him? If so, what? And also, is the fact that HE is chewing tobacco a threat to his family? For instance, if he kisses my baby sister or my stepmom can it harm them too? Please help, I wish I was more educated on tobacco...