What causes Harvard to rescind your offer?

[gibby]

I’m asking if the answer of having a real sickness is sufficient.

Yes, a documented medical leave of absence from high school for an illness – provided that your doctor AND guidance counselor can attest to the illness and write a letter to it’s authenticity – is a legit excuse for not completing your senior course load and will most likely not get you rescinded. Harvard might want further assurances that you are completely recovered from this illness before allowing you to attend the college.

If you don’t have a documented excuse from an MD and GC, well that’s another story: https://www.huffingtonpost.com/2015/01/20/avoid-college-acceptance-rescinded_n_6507536.html

It’s not just receiving poor grades that can lead to a revoked acceptance. Dropping a course or switching to a less rigorous one—for example, deciding to move from AP Art History to pottery your second semester—is also considered a violation of the “successful senior year completion” clause, Taylor says. Changing one class most likely won’t end with a college revoking your acceptance; however, if you were to drop all of your AP classes for joke electives, that’s a different story!

OP you really need to have a discussion with Admissions about this ASAP!