Flesh Allergy

• WWizards of the Coast2011-01-01
  Official Ruling
  If the targeted creature is an illegal target by the time Flesh Allergy resolves, the spell doesn't resolve. No player will lose life.
• WWizards of the Coast2011-01-01
  Official Ruling
  You can target a creature you control, then sacrifice that creature to pay the additional cost. However, if you do, Flesh Allergy won't resolve for having an illegal target. You won't lose any life.
• WWizards of the Coast2011-01-01
  Official Ruling
  Flesh Allergy counts each creature put into a graveyard from the battlefield that turn, including token creatures, the creature sacrificed to cast the spell, and the targeted creature (if it is destroyed).
• WWizards of the Coast2013-04-15
  Official Ruling
  You must sacrifice exactly one creature to cast this spell; you cannot cast it without sacrificing a creature, and you cannot sacrifice additional creatures.
• WWizards of the Coast2013-04-15
  Official Ruling
  Players can only respond once this spell has been cast and all its costs have been paid. No one can try to destroy the creature you sacrificed to prevent you from casting this spell.
• WWizards of the Coast2013-07-01
  Official Ruling
  If Flesh Allergy resolves but the targeted creature is not destroyed (because it has indestructible or enchanted by an Aura with umbra armor, for example), its controller will still lose life.

Rulings via Scryfall