What degrees involving science could lead to a job in international aid and development?

Thanks in advance guys.

I am in the process of choosing a degree path, and I have just recently been accepted to medical school in my home country. However, I am not completely sure it is what I want to do. I love the field, but I cannot see myself staying in the same place for very long. I moved quite a bit as a child, and I need to travel or I get antsy.

Anyways, I figure my best bet to truly enjoy my career is to make my passion my job - that is, I greatly enjoy travelling and being in challnging situations, experiencing foreign cultures and meeting new people. Ideally, I aspire to work for an organization or a private entreprise in the Middle East or North Africa in the area of security and development. That is, of course, years away in the future, but I am steadily gaining experience through volunteering and other work related experience.

While I definitely want to do something science related, I do not want to be in a lab. I love medicine, but I want to see the world and work in conflict areas. I could see myself as a liaison worker in the business field, or even as an environmental engineer. I am lucky enough to have a broad choice of programs, but that isn't making my decision any easier.

This brings me to my question: Which degrees offer great advancement potential, international demand and (I'm being honest) good pay in the field of science (particularly health, biology and toxicology or environmental engineering) that are still adventurous, exciting and people-oriented? I want to work for the UN, but that is every aid workers dream.

Any advice is appreciated!