in what kate did when sayid is getting tested for the sickness, dogen blows some dust on sayid i think this dust might have been the volcanic ash that keeps the monster/mib away. it seemed to not interact with sayids body (though i could be mistaken)if it didnt it seems that his body is repelling the ash because of the monsters influence, and the sickness. but i think as long as it doesnt spread to his heart that he will be safe. maybe it isnt literally his heart, but the meaning of the heart perhaps the monster has to influence you to turn you to the darkside. and maybe the electricity that they hooked up to his body was like the sonar fence and seeing that it hurt sayid was another way to know that he might be infected.