In general, you should write:
1) how you became interested in public health
2) what concentration you are interested and why
3) why you and the school you are applying to are a good fit
Here's a guide from UC Berekeley on the Statement of Purpose: http://grad.berkeley.edu/admissions/apply/statement-purpose/
I was verified on 12/3. I've only heard back from one school.
I'm glad you made this thread because I was getting ready to make one. It's been pretty quiet here.