Living Dead does it make you immune to poison

[ways and means]

Was just wondering does Living Dead come with immunity to poison (because of no blood flow), immunity to orbius and poison gas (no need to breath) and immunity to lots of environmental hazard which just wouldn't be a hazard to the undead. 

[wyvern]

Answer: yes-ish.  I'd handle this by aspect invokes / compels, though, since it's essentially a fluff-based thing - all of the particular effects you named could function mechanically (with appropriate fluff.)

For example, a salamander might have "poison" that injects a target with elemental flame - a form of damage-over-time that living dead would not protect against.  Orbius might be represented by a summoned constrictor snake, or an overgrowth of grappling vines, or even an exorcism type spell outright designed to halt and destroy undead.  Poison gas might be aerosol holy water, or ghost dust, or etc.

Assuming the thing with living dead is an NPC, I'd typically handle this via aspect compels against the PCs - things like "your poison doesn't work because the thing's dead; here, have a fate point."  If it's a PC with living dead... well, I could still do the same (if they didn't explicitly buy an appropriate physical immunity), or I could make the player invoke an aspect to get the appropriate advantage.

[sinker]

Most natural poisons rely on the blood flow to carry it's effect throughout the body, on the allergic response or nerve signals to cause harm, etc. Since a dead person has none of those things, it seems like most natural poisons would be completely ineffective.

Mechanically I'm not sure how I'd handle that. As Wyvern pointed out, you could handle it as an invoke for effect of their high concept, but I'm not entirely sure that it shouldn't just be part of the power that they paid refresh for. I suppose that would vary from GM to GM, and at least for me it would likely vary from situation to situation.

[Becq]

As wyvern said, but I don't necessarily think that it automatically qualifies as a compel.

I would tend to say that some attacks by their very nature only work against some targets.  For example, pickaxes and cyanide will both inflict damage against people, and pickaxes are fine for damaging a stone statue, too.  But cyanide will do very little against the statue, unless it's more or less a river of cyanide over an extended period of time or under very high pressure.  Similarly, most poisons that work by shutting down bodily functions could reasonably be said to have no impact on dead bodies (including BCVs, for example), and I don't necessarly think this qualifies as a compel.

Note that acids and 'poisons' that work by damaging/decomposing tissue should probably work normally, though Living Dead still protects against actual (re-)death unless the damage counts as one of the special means for destroying the creature in question.

[sinker]

Note that acids and 'poisons' that work by damaging/decomposing tissue should probably work normally, though Living Dead still protects against actual (re-)death unless the damage counts as one of the special means for destroying the creature in question.

Acids I can agree with, but poisons that cause necrosis are merely killing the tissue and then making it difficult for the body to remove that tissue and since the tissue is dead in the first place...

[Becq]

I guess I was thinking more in terms of a "flesh-eating bacteria", which probably counts as a disease, rather than a poison, but might well still have an impact on a living corpse.

But I think we're on the same page, in any case.

[Tedronai]

Or spider venom which might be better described as digestive juices.