What is public health?

This  is an extremely important question and oneas of late has created deep reactions inside me so this almost was posted on my other blog.
It is a valid question because some of the arguements i have had as of late revolve around this question and what a Health Department does.
The last heated discussion revolved over the role of a health department giving vacines.
To me this is a no briner, but was being challenged, so again the question what is public health?
I am looking for answers here.