Is Sound Constitution the only way to increase the overall health of a PC?

If you mean getting more wounds, then yes, I believe so.

If you mean actual 'overall health', I'd suggest buying up Toughness instead.

Maybe getting the mark of Nurgle, though that's not exactly improving overall health, just survivability

Yup. In Black Crusade, Toughness does not directly influence your amount of Wounds. Only way I know of is to buy Sound Constitution advances.

Oh and don't forget theres a single gift of the gods Corpulent Immensity (or something like that) with Nurgle which gives you an extra few wounds when granted.

Yeah, I think there's a few things that can increase/decrease Wounds, but Sound Constitution is the only "sure" way, afaik. But good catch.

...one day I will make that super-agile nurglite with corpulent immensity..

...one day I will make that super-agile nurglite with corpulent immensity..

aka "gasbag"

Just don't stand near him if he gets shot at!

[edit] Come to think of it, there was such a character in one of the 40k novels, even. He was some sort of psyker-mutant capable of breathing fire. iirc his buddies abused that by blowing him up to open a door.

Yes, as far as I know in most 40kRPG lines the only reliable way of getting extra wounds is Sound Constitution.

The only question is - why would you want to?

40k is pretty instagibby, so having extra wounds doesn't help that much. That xp could much more effectively be spent on Skills, other Talents, etc., and for 200+ xp 1 Wound is kinda a bad trade. I have almost always found that, unless bought in huge bulk at some kind of discount, Sound Constitution is one of the most useless advances in existence.

Then again I mostly play Tzeentchian types for whom the cost is vastly increased, so maybe it's just less appealing because of that.

The only question is - why would you want to?

Right now it's just curiosity. I bought Black Crusade recently and i finished reading it some days ago.

I was looking at the wounds of the demon-girl... thrice-possessed something. She has like a hundred wounds and no trace of Sound Constitution. That's when i started wondering about it.

After that i thought that maybe she took something like 2d10 from every demon consumed, plus some kind of ritual.

As far as I can tell, the back-of-the-book stats almost never list SC purchases, even if they have to have gotten them. Then again, those stats are misleading as to make them a threat without giving them Fate Points the FFG team normally gives them metric fucktonnes of additional wounds.

Also she's a Daemon Prince - we don't know yet what the rules concerning Daemon Prince wounds will be, and likely won't know till the next Tome finally drops.

One thing to quickly realise in BC, the ability to AVOID damage will be worth far more to you than the ability to endure it. There are so many "on contact" special qualities of weapons and likely the decay tome with Nurgle will probably have similar stuff. He is a god of disease, decay and poison stuff.

Additionally, NPC rules are always different. Don't bother looking for sound constitution or anything like that as they will probably not make any sense how they got generated just that it has been generated. See errata:

Question: There are several Adversaries in the Core Rulebook that do not seem to have correct Movement rates based on their Agility, Size, or other factors. Are these correct?

Answer: Yes, some pre-constructed NPCs can indeed have Movements, Characteristics, or other items that do not conform to the normal Character Creation rules.

While the question was asked about their movement it goes on to cover the rest of it. Characteristics here also covers wounds, stats and all that.