Higher education institutions, including medical schools, are required to provide equal access/accommodations for disabled students, and I am not aware of any school that does not have an office designed to provide/facilitate services for these students - this is not unique to Harvard. If your disability is not visible/obvious, and you can meet the technical requirements of the medical school with reasonable accommodations, you do not have to disclose your disability in the application process unless you wish to do so. If you do require accommodations, there will be a process in place (through the office for disability services) to request these, which usually requires a letter from your physician detailing the accommodations you need, but not necessarily disclosure of the specific disability.