Humanitarian engineering?

Civil engineering with a focus on hydraulics and hydrology, or sanitary engineering, is a good option. A large portion of the world lacks access to clean drinking water, or lacks water as a resource to grow food to eat.

You will likely have a career in a conventional engineering firm, doing work on infrastructure in a developed nation. You do the humanitarian work with organizations such as Engineers Without Borders, spending part of your year using your engineering education and experience helping improve the lives of those in other countries.

Perhaps biomedical?

Bear in mind that companies pay you to do work that will improve their profitability. You might do best to think of a money-making career and then donate your time and/or money to a humanitarian cause.

I think civil & environmental engineering would be most in line with what you want to do. That could easily lead into infrastructure development for the third world, etc.