Harvard is a major reach for any unhooked applicant. Their admission rate is around 5%. If you want to apply, then go ahead and give it your all. As discussed above your guidance counselor should note your illness in his/her LOR.
But also…Expand your horizons. If you don’t get into Harvard there are many outstanding colleges and universities where you can study neuroscience, have a great 4 year expereince and get where you wan to go in life. Spend the time and energy to come up with a well balanced college list that includes reach, match, and safety schools that appear affordable (run net price calculator) and that you would be excited to attend.