Unless you are willing to go to school and get licensed in some aspect of the medical field you will not be able to volunteer abroad doing anything remotely clinical. Sorry, but to do so would be setting yourself up as both unethical and libelous.
My daughter was a licensed Registered Nurse pursuing a BSN and a portion of her class was spent volunteering in Grenada for 2 weeks. Every student in the class was already an RN in the U.S. and they were not allowed to treat patients or do anything considered clinical. They visited various areas to advise mothers about nutrition, etc. but we're limited in what they were allowed to do. Be aware that every country has their own individual laws for practicing medicine that must be adhered to no matter how altruistic you happen to be.
If you truly feel compelled to help deliver healthcare in developing countries either go back to school and pursue a degree in the medical field or find the many other ways there are that are not clinical. You might start by volunteering at your local hospital or clinic. There are plenty of volunteer opportunities in almost every department where you can serve patients in other ways.
Anne Kusian, Past President of 900+ hospital Auxiliary organization