1. Weight is 99.9% of the time how much you eat.
2. The vast majority of overweight and underweight people thinks they are the exception (at least initially).
3. Health (the part of it you can influence) is almost entirely what you eat, whether you excercise or not, smoke or not, and your environement.
4. Underweight has its own sets of dangers and is about as dangerous as overweight, though is less obvious to the individual and those around him/her.
Edit: also, for those who like television shows, Supersize vs Superskinny is fairly good at presenting some of the dangers of each to the couch potatos out there. Season 4 is probably the better of the bunch. Here's an episode: