The most common issues involve various allergies, vision problems, asthma, or heart conditions, but the list goes on and on. The key is that any problems must be diagnosed by a physician or other medical professional and not "self-diagnosed" to be an issue.
If you have nothing on your medical record, and you pass your physical and vision test, you should have nothing to worry about.
Here is a sample questionnaire for DoDMERB. This can give you an idea of what to expect.
http://www.tamug.edu/corps/images/NROTCPHYSICAL.pdf
This form is completed once your SA application process passes a certain threshold. The SA (or ROTC program) will then open an account with DoDMETS to arrange for a government paid physical and vision exam (usually two separate appointments).
This sticky could be helpful too.
Over the years I have noticed a not so rare trend: A young person is treated in the military medical system and then when it comes time to receive DODMERB clearance they are disqualified for some treatment they received from a military physician. When this happens one of the first responses...
www.serviceacademyforums.com
One more point....while in high school, you should NOT be worrying about things beyond your control (like medical issues). The BEST use of your time is excelling at academics, studying for the SAT or ACT, competing in athletics, keeping in great condition, and participating in extracurricular activities that you feel passionate about. (I looked at your prior posts and I know you are very focused on these things).
Best of luck in your pursuit of a commission!