So if you're interested in working for the CDC, I will assume that you're interested in various areas of public health. There are several ways you can go about this:
1) Major in biology, become a PhD-bearing biologist, go join the CDC as a researcher. Your best bet would be to focus on a disease area which is of interest to the CDC. The most obvious are infectious diseases, such as influenza and HIV, but they also care about obesity, cancer, and other non-infectious diseases.
2) Major in biology, but pick up some math as well. Epidemiology (the study of how diseases develop and spread) is of huge interest to the CDC. (I have yet to find a field where knowing extra math is unhelpful.)
3) Become a doctor (bio major recommended) and join the CDC. I'm a little less clear on what the path is here, but a specialty in one of the CDC's top diseases, or public policy is a way to go.
4) Major in biology and public policy. This one I really don't know anything about, but have known people to start out their lives as biologists, and move into the domain of public health, the challenges of keeping a population healthy.
Hope this helps!