There is no doubt that the 530 bio score weakens your app…but no one here knows to what degree.
You can make a one or two sentence statement on your app that you were ill the day of the test…but if I were the AO I would be struggling to understand why you chose to take the bio subject test (your 9th grade science class), rather than your 11th grade science class which was either chem or physics.
I don’t know if MIT accepts March subject tests, but if they do, you might take it again.
I also don’t think it helps that you don’t have an AP science class in bio, chem, or physics. APES is not as rigorous as those.
I’m sure you know that MIT is a reach for everyone and hope that you have applied to a number of match schools as well as one affordable safety. Do you have any acceptances yet?
Good luck.