Flesh Allergy — ruling 1

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.

Flesh Allergy — ruling 2

You must sacrifice exactly one creature to cast this spell; you cannot cast it without sacrificing a creature, and you cannot sacrifice additional creatures.

Flesh Allergy — ruling 3

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.

Flesh Allergy — ruling 4

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).

Flesh Allergy — ruling 5

If the targeted creature is an illegal target by the time Flesh Allergy resolves, the spell doesn't resolve. No player will lose life.

Flesh Allergy — ruling 6

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.

Flesh Allergy : règles, fonctionnement et interactions (MTG)

Flesh Allergy is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

As an additional cost to cast this spell, sacrifice a creature. Destroy target creature. Its controller loses life equal to the number of creatures that died this turn.

How it works : Flesh Allergy

Original English text — Wizards of the Coast / Scryfall

#1July 1, 2013· Wizards of the Coast
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.
No other card is named in this ruling.
#2April 15, 2013· Wizards of the Coast
You must sacrifice exactly one creature to cast this spell; you cannot cast it without sacrificing a creature, and you cannot sacrifice additional creatures.
No other card is named in this ruling.
#3April 15, 2013· Wizards of the Coast
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.
No other card is named in this ruling.
#4January 1, 2011· Wizards of the Coast
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).
No other card is named in this ruling.
#5January 1, 2011· Wizards of the Coast
If the targeted creature is an illegal target by the time Flesh Allergy resolves, the spell doesn't resolve. No player will lose life.
No other card is named in this ruling.
#6January 1, 2011· Wizards of the Coast
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.
No other card is named in this ruling.