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Page 12 "Daemon Summoning" Daemons will only manifest next to an Icon of Chaos Undivided or an Icon belonging to a squad with the same Mark as the Daemons themselves" In the entry for Daemon Packs none of them bear any Mark but have all of the benefits of bearing a particular Mark. Common sense would say yes they do, but I need something more official. So for example do Plague Bearers bear the mark of Nurgle?

The common ones do not, but the Greater one count as an icon of their god for summoning.

And never apply common sense to GW rules. Never ever ever.

I think you have Chaos Icons confused with Marks of Chaos. A Chaos Icon is used for daemon summoning, a Mark of Chaos is a brand which can bestow powers upon its holder.

The Summoning entry is badly written. Daemons do not have Marks.

Ruleswise, they don't have them simply because the codex doesn't say they do.

Fluffwise, the Mark is a sign of the God's favour. Daemons are merely an aspect of the God in physical form. Giving them a Mark would be kind of like making the Christian God's foot a Saint.

So there is no way a Word Bearer army can field Nurglings

Sadly, no.
If I remember corectly they're allowed but since you can't get any marks of nurgle you can't actualy buy them. In effect a 0 - 0 choice.