To Blindluck and AspiringDoc,
Depending on the schools that you'll apply to, you most likely won't need any experience at all. The only schools that require more experience include Columbia, JHU, and Harvard (off the top of my head), but you can get into Columbia or JHU with some experience (like research) and strong GREs (meaning above 600s, probably closer to the 650-700 mark, in each section). For the most part, many public health programs are student friendly and have many people that are fresh from the undergraduate boat.
I went to Michigan right after college with a sociology/biology double major (added bio my last semester b/c I only had one class to take to fulfill the requirements), and there are plenty of people (esp in the behavioral sciences/health education) without science degrees. Biochem will not hurt you at all for epidemiology, toxicology, environmental health/industrial hygiene, or biostats (esp. if you've got a good deal of math background), even if you wanted to do something related to health education (I had a friend in my department that did a Chem major).
GPAs for public health programs can range from 3.0 to 4.0, meaning that there is no single GPA that will definitely get you into a specific public health program. I'd assume that a higher GPA and GRE scores would help at a so-called "top notch" program, but I'd think that anything around a 3.3-3.4 would be more than sufficient. I mentioned in another thread that I know people with different combinations that have gotten in - high GPA/lower GRE (meaning below 600, which is not bad) or lower GPA/higher GRE (closer to 700), and some in the middle of both categories. It all depends on the person. Of course, any experience that you can bring to the table is always helpful, but don't feel that you have to have public health experience to get into a public health school. Not many people are lucky to have an undergraduate course or major in public health are their schools, so it make sense that most schools wouldn't expect that you would be completely familiar or experienced in the field. I think that a showing of your genuine interest in the public health field will be very helpful in determining whether you gain admission (i.e., have you at least pursued avenues that show some sort of interested in public health, or researched it to the extent that you show knowledge of pertinent public health issues in your personal statement?).
I guess the inability to find adequate resources on average scores at public health programs, etc., mainly illustrates what I said above - that there is a broad range and each program tends to look at you as an individual - with your motivations for wanting to obtain a Masters degree in Public Health being one of many important determinants of whether or not you get in.
Best,
H&T