Public Health is a group of integrative and interactive disciplines working to improve the health of all.
During my undergraduate career I volunteered in the communities of Trujillo and Agallpampa, Peru and fell in love with the experiences I had helping people through parasite prevention, education and establishing free weekend health clinics.
The single most rewarding experience of my career was seeing the immediate appreciation and changes in the communities I volunteered in during my time in Peru. The individuals there were happy to see us and were grateful of our efforts. My experience with the people in Peru influenced my view on global health by giving me the realization that even small interventions can have great impact.
Not to narrow your focus in the field. There may be a specific concentration you are initially drawn to or a certain area of interest you would like to pursue but I would challenge you to find several areas of interest in the realm of public health and find a way to incorporate them to make your own impact in the field.
I think the biggest challenge in the public health field is the integration of vertical and horizontal health programs. In order to take full advantage of the two systems there needs to be a middle ground where the benefits of each program can be maximized.