Rather than give you any misinformation on it from my own opinions, there is a website from which you can start your own investigation: American Medical Writers Association http://www.amwa.org
Check out the Links section also. It lists several other science related writer websites and organizations. If you contact any of these, explain your interest and I would imagine they would be very glad to help you learn about their trade. They would also probably have some good suggestions on how to proceed in preparing yourself. If you are lucky, perhaps someone will help mentor you, giving you advice on a regular basis as you go.
As far as the education needed, a PhD or masters would be great but I believe a bachelor degree in biology, chemistry, business, or computer science would be sufficient. That would be the background, and then for each article, corporate product description, medical brochure, science textbook, owner's manual, or whatever your assignment, you will do further research. It might be possible to get an associate degree in biology or science and an associate in journalism and that could be sufficient to get you started but it is more the writing practice you do on your own that will help you become a better writer in explaining ideas. That's why I suggested some fiction writing and perhaps join a local writers' group where they meet weekly to critique/encourage each other.
Get as much education as you can but, i feel it is not how many facts you can remember, it is that education shows you how to learn on your own when needed, how to present ideas (written or oral) when you have to, and how to work persistently and efficiently on projects to achieve goals. As a technical writer, you would get to go into alot of different areas of science or business whereas the typical lab worker or business worker works on the same topic everyday, month after month. Technical writing is probably a more secure future than some areas of science where you are competing for grants on a regular basis.
Also check out some colleges to see if they have a major in technical writing or suggestions. Another suggestion is to try your hand at writing a short article on MS and see if you can get it into a local newspaper. You will at least learn more about the disease from the research.
Hope that helps. Wesley