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Been mulling over an idea for a unknown xenos species and wanted to know if something similar already exists in the background. The premise is a xenos species that is spore or bacterial form, that either leads to mind controlled subjects or even changing the hosts brains/bodies into anew life forms? Doesn’t need to be a hive mind per se, and perhaps the spores give a boost to strength and speed to the hosts as well?

The other aspect was this bacteria can almost spread similar to how an idea is spread? So instead of it being transmitted through the air, it passes from person to person just through thoughts? Very crazy I know but it would interesting to have any enemy that multiplies/reproduces through thought over more tranditional methods. Any help is appreciated.

Sounds like the flood from Halo. In 40k the process sounds similar to chaos corruption. Talk about stuff and as ideas take root your soul starts to be damned. In extreme cases mutations might come about. If you have a deamon spreading propaganda it might actually place itself in peoples minds that have heard of it, whispering temptations to them and whatnot. I get that your idea is more of a biological one rather then the influence of a different dimension. What doyou intend to use it for? A basis for an army, roleplay or just your own storylines? Whatever the case go for it. Sounds like a neat idea.

I don't think all the items you mentioned are covered by one species, but I think they are each in the lore somewhere:

Spores/fungus -
Orks
Tyranids

Mind controlled subjects -
Genestealer cults
Possibly Rangdan
Enslavers
Arguably Tau (ethereals have an uncanny level of control over their populace, IIRC its either unexplained or pheromone based rather than psychic. Not sure).
Chaos corruption can be very similar to this too, especially manipulation by Tzeentch.

Chaos corruption very much sounds like it fits the bill for passing corruption based on thoughts, especially if thoughts means ideas, beliefs etc.

Enslavers/Hrud/Rangdan might be worth looking into.

Brainleaf is a thing, basically a plant that mind controls victims and turns them into essentially zombies.

Thanks @Nerak, to be honest I was thinking of using this as a basis for the Rangdan in some short stories I am working on.
Yes I agree it does sound demonic or metaphysical in a way, was also thinking of the mind control aspect associated with the Rangdan. All of this came from a re-watching of occulus imperia’s YouTube video on the Rangdan xenocides, and he came up with the idea before I did haha. Haven’t decided if it will be a strictly fungus/bacteria race of xenos, to have varying or specialized groups all apart of the Rangdan species, or if exposure to the bacteria changes the infected over time to different creatures. Since they also have the ability mimic imperial ship signals and are known for surprise attacks, being able to read and absorb thoughts seems logical. Also thought it would interesting/cool if there are specialized infiltration soldiers that can shape change like The Thing or T-1000 from T2.
This could also explain why they were so difficult to kill and needed the Dark Angels and Space Wolves to basically massacre entire systems. If you can’t tell who is infected, just order exterminatus.
From a story perspective I want the Rangdan to be a mirror to the Imperium during the Crusade. Massive technologically advanced society that wants to destroy all other races for self preservation. Thinking they way the Rangdan reproduce is by taking and mutating children to make new soldiers, like the space marines alter human children. But still wanted to know if there had been anything like this before in previous editions or stories.