sgilani,
Depending on your current major/interests, and the program you'll apply to for Public Health (i.e., Epidemiology, Health Education, Environmental Health), experience in PH is helpful, but will not make you less attractive as an applicant.
I already have my MPH and when I entered the program at Michigan, I didn't have any experience in Health Education (which was my concentration there) besides an undergraduate public health course. Basically, I wrote in my essay about how I would be an asset to the school in terms of the skills learned in other undergraduate jobs (tutor, student assistant in various offices). Just try to finesse your skills and you can write a great essay based on your limited experience. Also, it would help to apply to student-friendly programs (which seems to be most public health schools) that don't take limited experience into account as long as you've got a genuine interest in public health. I think that Harvard is definitely a school that doesn't accept folks without a terminal degree, but otherwise, you've got a great shot at many schools.
If you need to bounce off a few ideas, feel free to PM me.
Best of luck in your writing.
H&T