Permanently or as a short-term relief worker? One may be more difficult than the other. It will require licensure in the country where you'll practice. The hoops you'd need to jump through to be licensed may depend on the country's desire to protect it's job market for local physicians vs an unmet demand for clinical services.
Here's a link to Doctors Without Borders just to give you an idea about one route to practicing abroad short-term.
Before you apply