Once upon a time…

Once upon a time there was a single-celled parasite that had infected the brains of billions of humans.

It knew a lot about how the mind works, so it influenced the hosts mind not to see it. It could manipulate fear-response and influence cognition. It could so because it had itself developed over millions of years, along with the development of the brain and central nervous system (CNS).

As it was alien to the human body it carried with it a large disease burden — both physical and mental.

One day Nature decided to reduce the impact of the parasite, because it had caused an imbalance to grow beyond a threshold. So, Nature inserted a virus that would attack the parasite. Many viruses have that objective; to create and maintain a delicate balance in the microbiome.

Obviously, the parasite had to act quick and mount a defense. It had to, at any cost, to prevent the host from being infected by the virus…

In real life we have something that is eerily similar to this, its name is Toxoplasma.