Working on a story of an alien body-snatcher invasion type of story. Basically this is a parasitic symbiotic species that evolved to reproduce through their own microbiome cloud by air and touch. Female "symbiotes" in their host bodies spread their microbiome cloud which is inhaled or absorbed through contact by people and other beings, which then lays a growth of microscopic eggs in our brains. Then male "symbiotes" in their host bodies spread their microbiome clouds through the air, which then fertilizes these eggs after it's been inhaled or absorbed.
[EDIT: I have decided to drop my original pheromones idea and adjust to using our own microbiome clouds, which we all have around us in the air and on our skin, full of bacteria and viruses and more. I will have these aliens produce their own microbiome clouds which attacks the personal microbiome clouds of organic beings which then replaces our personal microbiome cloud with their own, which carries their reproductive DNA which we then inhale or absorb through our skin, which then enters our bloodstream and "infects" our brains.
Thanks for all the comments, although absolutely nobody had any answers on how to make my concept of infection by air and contact work, I am still glad that all that salty criticism has led me to an alternate concept that works better than pheromones which could be more palatable to discerning readers than pheromones.]
When these airborne alien DNA are breathed in or absorbed by contact, it enters their bloodstream and travels to the brainstem and enters our brains in this way. And then, after fertilization, the alien growth begins to grow and begins to consume and absorb the nutrients of the host brain as it grows. Eventually, this growth consumes the entire brain and becomes its own newborn intelligent alien sentient brain which then lives in and controls the body as if it was its own body. The hosts brains are completely replaced and the hosts are in affect dead, although their bodies still live on with alien brains. This process is not reversible in any way. This is how the alien "symbiote" lives, as an alien brain that lives through their host bodies after they replace their entire brains. The body gives them the nutrients and proteins necessary through normal bodily means and so the body does not die and the alien brain continues to live on.
These "newborns" are very often like amnesiacs who need guidance when they're "born" but they always share a basic ancestral memory of their species' origins starting from their homeworld and the start of their spread across the cosmos. So they are always instinctively self-aware and driven to help their species and Empire grow.
They just don't know how to live in the bodies they now occupy unless they can learn. They do not even know how to speak in any language unless they're taught. Often they will befriend natives to help them and teach them their way of life and language and how to use their technology and so on, all the while infecting their newfound friends, unless they are on an world they already conquered. Then, they have "families" that take care of them and to teach them when they're born.
Is it implausible to use pollinated MICROBIOME CLOUDS as their way of reproducing?
Their means of reproduction is beyond their control. It is simply their biology and so every host species they live in constantly exude and emit their pollinated microbiome clouds because their alien biology replaces the host body's sweat glands and produces their own microbiome cloud through their host body's system.
Since the beginning of their species, they have always thought that it meant that all species were destined to be a part of their Empire.
They do not understand how they are reproducing in other species, in the beginning their species were actually confused and perplexed at first every time they made first contact as they began exploring space, but now they simply just believe that this is the way they're meant to exist and that it is their divine destiny to conquer every world and for their species to live on within every alien species they come into contact with, ever since the time they left their homeworld and started their empire.
They don't abduct or kidnap people to infect them, they have no need to force themselves on a host species, it simply happens. As it is biology that they do not truly understand yet or even control, they are not trying to force their species on others through evil means. Just being around others is enough.
As such, uncomprehending, in the beginning they meant no true inherent harm to other species when they began to explore space, but because they don't know how their species causes "infection" or how to prevent it and it inevitably always happened with every other intelligent species they came into contact with, they began to believe it was destiny or fate that all other intelligent species would be a part of their parasitic/symbiotic species and Empire.
Even if they don't understand it, they are always aware of their origins and that they are "infecting" other species. Even though, they are still "newborns" when they're "born" that need to learn how to live and survive after being born and will develop their own personalities and individuality as they get older.
They are not inherently evil, although the way they reproduce seems sinister. Since this is only the way they've existed since the beginning of time for them, they believe they're improving the universe and that it is their destiny that their species will live on in all alien species throughout the cosmos.
Now, however, after thousands of years and building their civilizations, they now use their military might to invade unsuspecting worlds instead of coming in peace because their culture now believes in their divine right to do so.
They do not retain their host body's previous memories. Although they are not a collective hive mind, they do follow their collective base instincts: to preserve and continue their species. So even as they're aware that they are changing everyone they meet, they will continue to do so because their instincts drive them to continue, even if some feel guilt for doing so. They know that they could choose to quarantine themselves from "infecting" new species and new worlds, but they choose not to.
Most knowledge they have is secondhand, they are not born knowing everything.
They are a singular invading alien species that live in hundreds of different alien species that they control within their Empire which has been traveling the stars for thousands of years and have conquered countless worlds, and have discovered Earth and are now invading Earth with their armies, where the alien soldiers' constant exposure around humans on the ground is changing us even as we try to fight back against the alien invasion.
So this invasion is twofold. By attack (their empire is vast and still needs planetary resources so invasion in order to plunder resources make sense), and by infection to further ensure the survival of their species.]