Medi-pack
Medi-packs (also referred to as Narthecium) allow a unit with a medic to ignore the first failed saving throw it rolls in every turn. The medi-pack may not be used on any model which suffers instant death (see the Warhammer 40,000 rulebook) or that has been hit by a close combat weapon that allows no Armour save. The medi-pack may not be used if the medic is in base contact with an enemy model.
Errata — Chapter Approved: Codex: Imperial Guard Q&A (v1.0): A medi-pack can be used against a wound caused by a shooting weapon that allows no save at all (such as a heavy bolter's AP5 against a 5+ save model) — the only wounds it cannot ignore are those from a weapon with at least double the victim's Toughness, or a close combat weapon that allows no Armour save.