Most people recommend refraining from eating bananas and wearing anything with a scent (perfumes, laundry detergents, etc). Following this, I've been pretty lucky when when travelling aronud Mexico I have ALWAYS worn repellant, especially if visiting archaeological sites, cenotes (or other standing waters), small towns, etc. Basically, any place not touristy.
As for malaria - I haven't ever had any issues and have visited and lived down there - all over the Yucatan. However, I have had "montezumas" which might be water related or due to overexposure to the sun. Regardless, the Mexican pharmacies don't really require prescriptions to get prescription medications. Its great if you get sick - just remember if you have any allergies.