I really doubt a "mirror life" bacteria would be "hidden" from the immune system. Antibodies should have enough search space to be able to detect and neutralise such bacteria?
Most of the key molecules of life are chiral, existing in one of two mirror-image structures; life uses only one. Receptors and enzymes using one cannot generally recognize the other. Most 'wrong-handed' chiral chemicals are like having a key cut in a mirror image where the index slot and the pins are cut on the wrong side; it won't even fit in the lock.
It could plausibly turn out there is so little ability for the different chiral forms to interact that 'mirror life' would not be a threat. It could also turn out to the extent they could interact, regular life could also detect and evolve how to counteract the novel pathogens in time.
And even if the interaction is low so they aren't highly infectious, the lack of ability to recognize the 'mirror-life' means regular life cannot attack it so mirror life grows unchecked and consumes resources needed for all life.
But there are so many unknowns, and so many unknown unknowns, and such absolutely catastrophic consequences for life on earth, that relying on ALL of those possible success modes working is stupidity beyond insanity.
This is probably the biggest example of "just because you can does not mean you should".
more seriously: how would it be able search for something it has no experience seeing at all
In one example, the mirror molecules would likely cause some sort of localized inflammation or other lesion, and then the immune system would be attracted to the area, generating trillions of combinations of randomized protein sequences. If within that trillion there is just 1 antibody that can bind to the mirror life, the immune system will duplicate that molecule many times, and they will bind to the mirror molecules, recruit other proteins that will isolate and degrade them, and then remove them from the body.
Due to the complexity of protein structures, antibodies can bind to mirror molecules.
Every time I show a computer scientist this system: https://en.wikipedia.org/wiki/V(D)J_recombination they remark, "Wait, nature uses self-modifying code? It generates trillions of random combinations, shuffles them around, finding the best ones, and then amplifies them? You know that sort of sounds like this CS technique called genetic algorithms" at which point you just stare at them until the exclamation point above their head goes "BINGO"
With perfect knowledge of the immune system and the capability to create the pathogens of our dreams it would most likely be possible to create this pathogen of our dreams.
Making an entire self replicating mirror organism that would both have this property of "being invisible to the specific immune system" and AND being an effective pathogen is far from a solved problem though. It's further away as useful quantum computing!
We don't even have self replicating mirror cells, just some synthetic Frankensteins.
A reasonable argument is that mirror proteins will evade the usual degradation+antigen presentation pipeline and this will make efficient adaptive response more difficult, or that the innate immune reaction will not be enough to recruit adaptive response. But surely it's not impossible. And if we ever get to this point I'm very optimistic that we would have a good way to vaccinate against this.
We just don't know yet, but I want to believe because I think it's exciting. Protease failure, transport machinery failure, MHC binding failure, they are all stereospecific after all... So it's possible.
> And if you can do that then why go through mirror life anyway?
Because our body can produce antibodies against all "classical" forms of life. It's probably impossible to produce classical pathogens that are fully invisible to the immune system. That's the whole point and danger of mirror life in the context of pathogen creation. Plenty of classical pathogens were our antibodies (usually) don't help already exist though: HIV for example, where only very few people are lucky enough to be elite neutralizers. If we wanted to do biological warfare, it would probably easier to enhance those viruses.
> And if we ever get to this point I'm very optimistic that we would have a good way to vaccinate against this.
I am not so sure about this, if the bodies molecular machinery is unable to produce antigens for those pathogens, no classical or mrna vaccine is gonna trick the body into producing them, it's just technically impossible. You'd need to "update" the whole immune system to be able to manufacture the mirror antibodies and from that we are an eternity away. The only remotely feasible cure in such a world would be through infecting the body with another mirror-pathogen that produces mirror-antibodies or directly injecting mirror-antibodies...