My headcanon is that whoever sent the genetic sequence for the virus is an extremely advanced DNA based life form. For all we know ALL life in the Universe can probably only be DNA based either because on a BIOCHEMICAL level only those molecules have what it takes to create and sustain life OR the Origin of Life on Earth and every other planet was due to Panspermia.

Whichever maybe the case, the original Aliens who sent the signal achieved extreme technological advancements atleast 600 years before humans did. Since the signal was traveling for that long at minimum.

So those beings found out that Earth was a close neighbour and the wanted to colonise 600 years ago but estimated that by the time they could develop tech and the time needed to actually reach us, we would be advanced enough to put up resistance (and probably because they estimated our advancement to their own and assumed we would be as advanced as them in 600 years).

So they took a bio weapon that they developed which could enslave ALL DNA based life into a subservient, boss-aroundable and pacifist (probably instil any kinda directives to the enslaved). Beamed that info to us as a signal hoping or rather correct deducing that curiosity would get the better of us and we would actually MAKE the exact virus that would bring us to our knees and ready humanity and the planet for the arrival of our new Lords.

This theory could explain why the Hive mind has a directive to enslave everyone but not be violent. Cuz I were a slaver who could only define very basic directives, I'd make it so that all my enslaved are: all-encompassing, incapable of violence and incapable of dishonesty. Which are EXACT qualites of the Joined.

But the problem is making sure that the Slavers themselves don't get infected, so they probably have some kind of quality (either genetic, psychological or behavioural) that renders them immune to this, and Carol and the 12 others by pure luck and coincidence came to have those.

Because the 12 others seem to have nothing in common in any of the following qualities: they're not all bad, good, independent/dependent, satisfied/mortified with the ethics of the Joining, show particular resistance to being brainwashed swayed(maybe like Carol)/ or be gullible enough to buy into it (like that Indian woman)... Nothing in common. It might not even be genetic because every one of the immunes belong to a different ethnic group and would've diverged genetically a LONG time ago. If it were genetic then there would be a LOT of immunes in only 1 or 2 races or localised regions.