I gave a similar assignment in my English III class last Fall and will give you some ideas and advice of things I would have done the same and differently.
First, I had them list different health topics, diseases, and/or issues that interest them or affect them personally. This was a good start. Part of my problem, however, was that I didn't want them to simply write expository essays (or book reports). I wish I had made them take their initial issue/topic and then done preliminary research to come up with a controversial question about it. I would have had them pose an open-ended controversial question and research each side. As I think about it though, this would have been much more of a scientific assignment than an writing assignment, and would have required a lot of reading and research in fields in which high school juniors would have had very little experience. In short, it would have been very difficult.
Other ideas I brainstormed in retrospect were for me to come up with various health debates (current and relevant to high school students), created the controversial questions for them and provided this list from which students could choose the topic for their papers. I think I would have come up with different behavioral/psychological issues, current debates on child/teen health issues, and questions of the effectiveness of certain types of treatments over others. It really seemed my students did not have a big grasp of the existence of such debates in everyday life (not just among doctors and scientists).
One big issue you may want to consider: sources. Do your students have access to a scientific library (or an online database that is more reliable than easy-to-find websites and wiki's)?
My students were all very interested in their topics because they all knew someone suffering from the disease or health issue they chose. This was a positive. On the negative side, the essays tended to read more like basic book reports and re-organized versions of the various information they had gathered, which, in itself was fine, except not as challenging as I would have liked.
I always find research papers a challenge, even with the brightest students. It seems that most students (high school and college from what I've heard and read) really struggle with the ability to argue logically, and in writing.
Would love to hear of your struggles/successes as you come to them!