Chrise, I don't think "endemic" is correct. The dictionary.com website describes endemic as "belonging exclusively or confined to a particular place" which would seem to be the exact opposite of of widely prevalent. I'll look at a different dictionary to be sure, though.
Endemic fits in with all words around it! However I'm stuck on the right lower area. 15A make a further offer ( 5 letters) E???D
22A attained (7 letters) R?I???D
27A.Add money to account ?E???T
16D. Conducted oneself (7 letters)