Hi, Congratulations on getting a 710! . First of, it's an outstanding score. the fact that you had a viral fever on the test day and still could score 710 is astounding. I think you fare pretty well as your score in somewhere in between the range. don't worry about average demographic scores, GMAT is just 1 aspect out of several.
If you're asking about how to respond in your optional essays, it might be something like
"I understand my GMAT score is below the average demographic; However, I had viral fever the day prior to sitting the test. This was the only time I could sit the test this year owing to personal circumstances. I am happy to to accept an offer that is conditional upon me achieving a higher GMAT."
I've read somewhere else that people have been given conditional offers to b school but you must improve the score, and I'm confident you will. Goodluck