Codex: Chaos Space Marines (2012), p30 — Mark of Nurgle

Nurgle's blessing manifests not only as a suite of virulent plagues and diseases, but also as the inhuman constitution needed to survive them.

Models with the Mark of Nurgle have +1 Toughness. Additionally, a Psyker with the Mark of Nurgle must generate at least one of his powers from the Discipline of Nurgle (pg 71).