I would add that bear attacks are possible in most of the national parks. Here in Florida black bears routinely come into populated areas. In the western parks there is also the possibility of mountain lion attack although these are quite rare. Any large animal can be dangerous. People here in the US and Canada have also been attacked by elk, moose, and bison. However in most cases it is the human's fault. Always announce your presence by making noise and avoid females with young.
There are excellent photographic opportunities in our national and state parks. Just use common sense and stick to marked trails. Always follow a ranger's advice and read the signs.