The magic in "Final Fantasy X" often has the ability to cause a status effect on members of your party until you take measures to cure them. The Zombie status effect turns the members of your party into the undead, which causes them to become weaker to holy magic and take damage whenever they use healing items and spells. Removing the Zombie status effect requires the use of special items and spells as your normal options for healing will do more harm than good.
Use a Holy Water to cure a single party member of the Zombie status effect. Holy Water can be purchased from shops for 300 Gil and is often found in treasure chests throughout the world.
Use a Remedy to cure a single party member of all negative status ailments, including Zombie. Remedies can only be purchased from Rin's Item Shop on board the airship, or found in multiple chests throughout the world.
Cast the spell Dispel on a single party member to remove the Zombie status ailment. Any character can learn Dispel, however Yuna is the first party member to do so.
Wait until the enemy kills the party member afflicted with Zombie and use a Phoenix Down to revive him. When he returns, he will automatically be cured of the Zombie status ailment.