It's really great that you're trying to plan ahead! Let me give a brief overview of when specialization occurs to try to steer you to what you actually need to worry about now as a potential pre-MD/PhD student.
Medical school is like a world fair where you sample the sounds, tastes and colors of a country for a few minutes before moving on to the next. You learn the basics of the different parts of the human body and do clerkships in the "core" specialties, with a few electives of your choosing. These clerkships help you decide what residency you want to do after graduating. Residency is like moving to a country you checked out at the world fair and getting to know the customs of culture up close and personal.
Check out:
http://www.abms.org/who_we_help/physicians/specialties.aspx The left column shows specialties you can specialize in. The right column shows what you CAN subspecialize in after you complete residency in a fellowship program, although it's optional.
After residency one has the option to specialize further in fellowship. This is like getting married, having a kid and really settling in and knowing the town in another country after living there for a while. Some residencies, like internal medicine, are very broad with many subspecialties (including endocrinology), and some are pretty narrow like ophthalmology with no subspecialty (you can still do fellowships however). During your residency you may still do rotations like in medical school, but this time you are trying out the various subspecialties. Sometimes there are many specialties/subspecialties that touch on an area of research; for example if you're interested in immunology you could conceivably work in Allergy/Immunology, Rheumatology, Oncology (eg studying tumor immunology), Neurology (eg studying autoimmune disorders), etc.
Graduate school goes about things in a slightly different way. Say you wanted to learn how to make the best stew in the whole world. There are many countries that make different type of stews, and in each one are several master chefs whom you can choose to learn from. So how do you go about it? Well you need to choose a country that has the type of stew flavors you like, but you also need to choose one with master chefs who are good teachers and who are willing to teach you. You might need to try to learn from several chefs first before settling on one to work with, and even after learning everything your mentor knows, you might want to learn from someone else to blend what you're learning into the newest tastiest stew ever.
So the flavor of stew is the type of research you're interested in: neurology, endocrinology and immunology. You want a graduate program (country in my example) that allows you to take classes in those areas, and allows you to work with PIs (chefs) in those areas as well. Ideally you also want those PIs to be really good at what they do, and really good at teaching students. Since you're interested in a lot of topics, that means applying an umbrella PhD program that has researchers in a lot of different fields rather than just a pharmacology program or immunology program. You rotate in labs working in fields and pick one lab to do your PhD thesis in. Sometimes you won't find a PI that does research in all three things, or you may find a really great and kind one that studies 1 or 2 of them, and a PI that's a busy jerk who studies all three. Go with the nice PI. At first during your PhD you'll just be doing a project within that person's realm (following the chef's recipes); but with the tools, techniques and scientific thinking your PI teaches you, it's common and appropriate to branch out into what you really want to study, often by first working with another PI that can teach you the other areas you're missing.
So what does that mean for you? Well for one, you don't have to make this decision until pretty far down the road. Two, the reason for that is because you can't really prepare for what is a "good field" for you - you have to try it out for yourself.
Three, trying things out for yourself doesn't just start in graduate/medical school. In order to get IN to these schools,, you need to have some experience with medicine and research to see if you like it, and also with physicians and researchers to get a better idea of what they do on a day to day basis.
How do you go about doing that? Well my recommendation from reading your other posts is that you need to transfer to a research university that has an affiliated hospital. There you will have access to resources you need to understand and prepare for the application process: a pre-health advisor, research opportunities, physicians to shadow, including MD-PhDs. These people will not only teach you about their profession, they will also advocate for you in your application. Depending on your support network and how far you're willing to go, I would particularly look into universities that already have strategies in place for blind students. You could also consider contacting University of Wisconsin Medical School since they already have accepted and graduated a blind student relatively recently, on advice for getting into their program in particular and what kinds of tools and accommodations they used to give Tim Cordes the training he needed. The hospital may therefore be an easier place for you to get shadowing experiences than many other medical institutions. In addition I would contact Timothy Cordes himself, who is now an MD PhD (a psychiatrist and biochemist) at University of Wisconsin. Since he already went through university, medical school, and graduate school, he would be able to advise you on the tools and strategies for success as a blind student, how to do research in a lab, how to shadow a physician, etc. He could potentially even mentor you as a research advisor since he did interesting work representing proteins musically instead of visually.
Good luck, and hope that helps you put your planning energies into good use!
