An organism, say a virus or a bacterium, or maybe a parasitic worm or even a fungus - it really doesn't matter other than having to think up some way to pass it on - develops the ability to control human minds. This organism then depends for its success on making it's host determined to infect anyone and everyone they meet, including their own children, by any means, in the belief that being infected is normal and being free from infection is somehow abnormal, even dangerous - so dangerous in fact that infection-free individuals should be denied basic rights and even killed if they won't become infected.
Part of the parasite's success would need to be a complete disregard for the welfare of the hosts so they were prepared to die just to help spread the infection or to prevent the cure being applied to sufferers. This would be greatly facilitated if the parasite included in it host's mind control the idea that, just so long as they obeyed the needs of the parasite without question, they would eventually be rewarded and made immortal so they could have eternal life even after physical death. They would then feel invulnerable, even eager to die.
Imagine the chaos which would ensue as infected people began to gain control of society, of the laws, of the education system, of the legislature and judiciary, of the military complete with its nuclear arsenal, and began to threaten other societies.
Imagine the carnage which would ensue as wars broke out between carriers of the parasite and non-carriers. Imagine too how the carriers of different forms of the parasite would turn on carriers of other forms as the parasite evolved and developed new local strains which came to predominate in different parts of the range. Imagine how, as more and more people became infected, the society they had to put up with became more and more designed not for the welfare and well-being people but for the parasite, and became subverted for the propagation of the parasite itself.
How could humanity prevent itself being completely taken over by this parasite? The cliffhanger to the plot would all depend on whether scientists could come up not just with a cure, but with a mechanism for delivering it to reluctant victims, who would be fighting for the parasite and resisting the cure.
A tricky problem. Imagine the huge relief the reader or audience would feel when science finally triumphed and freed humanity from an eternity of slavish obedience to the parasite, like mindless automatons. What a wonderful ending that would be. Tragedy averted and mankind saved again from a fate worse than death.
A new Independence Day.
But is this plot realistic? Could such a parasite evolve and would intelligent beings allow themselves to become infected by it?
Did I include memetic as well as genetic organisms in my list above?
Memetic organisms are rather like genetic organisms in that they rely for their propagation though time on the ability to replicate. They don't exist in physical form but only in the minds of intelligent beings. Memes, like genes, are units of cultural inheritance and form alliances with other memes to form memeplexes.
Like viruses, which lack the mechanisms needed to replicate themselves and so need to take over control of a cell and use it's chemistry to make copies of themselves, so memeplexes can only replicate if they take over a culture and use its mechanisms for reproduction to replicate themselves.
Unlike genetic reproduction, in which new individuals are born with their compliment of genes, memetic reproduction depends on people acquiring them after birth. This is why we inherit our cultures not just from our parents but from our peers and authority figures in our society, and why human cultures tend to be highly regional, often language-based. The main reproductive mechanism memetic parasites subvert is the mechanisms we use to replicate our cultures in our children and so pass them on to the next generation.
Oh! I've just realised. I've described the takeover of human culture by the memplexes we call religion and the current fight to fee ourselves from it, haven't I.
I wonder if humans or the parasites are going to win.
Surely, with probably the cleverest brain ever evolved, certainly on this planet, humans aren't going to let it be taken over by a parasite to be used to replicate the parasite rather than for the benefit of human beings, are they?