Let's talk aliens.

[Treewyrm]

I didn't say anything about thickness actually, not ever mentioned that in the context, also I've not said that they cannot have infection, what I have implied is that it would not propagate through the rest of the collective and would be isolated to individual being. Considering their technology goes as far as creating veils and controlling them such disease if happens is likely to be cured as well, perhaps may take some time, and if not by being itself then through the help of others. I suppose Nomads' biological shell is quite responsive to attacks of that kind.

Would a Nomad attack a giant group of humans, knowing that it would die in the process, or try to kill them, but run away if needed?

Death and quasi-death subjects are extensively covered in the lore. In terms of behavior the nomads are willing to take up bigger risks, which sometimes could be mistaken for "suicide runs" by humans. Nomads are quasi-immortal, but the death toll is partial memory loss, meaning they might loose valuable information or some personal set of skills. Here is a simple analogue: think of it like of any hack'n'slash RPG - your character cannot truly die (unless in "hardcore" mode) but the game punishes you for dying by either subtracting character's experience, money or both. Similar happens to the nomads: they can't really die, but they wouldn't suicide because it's a bad idea and cases when they would do so willingly are all too rare.