I will reply in multiple posts The Precursors are beyond time and space and the Primordial even survived the Halo foreign on him when it freed him. They have existed before the universe itself. The Gravemind tells us something impossible to understand— that most of what has been gathered comes from before there were stars. We do not believe in such a time, but the Mind insists … The life-patterns and living wisdom of a hundred billion years. Bear, Greg (2013-03-19). Halo: Silentium (Forerunner) (p. 322). Tom Doherty Associates. Kindle Edition. Source https://forums.spacebattles.com/threads/precursor-flood-source-thread.264267/ So that is why I said they are immune.