The fungus Ophiocordyceps unilateralis has a macabre life-cycle. It infects carpenter ants and takes over their brains, controlling their behaviour so that they leave the colony and find a leaf at the right height, temperature and humidity in the forest canopy. There they crawl round to the under-side and bite into the central vein. Then they are killed by the fungus which has been consuming their body.
The fungus then sprouts a fruiting body from the head of the ant and releases spores which fall to the forest floor to infect other foraging ants. The bite mark on leaves heals to leave a characteristic dumbbell-shaped mark on the underside of the leaf. These characteristic marks have been found on 28 million-year old fossil leaves from Germany, so this parasite had evolved at least 28 million years ago, even before the Himalayan Mountains had formed.
It's not just fungi that can perform this trick either. A parasitic flatworm called Dicrocoelium dendriticum does a similar thing. I'll let Oatmeal explain it.
The video on the left shows this behaviour in the unfortunate ants.
So, that's fungi and flatworms which can perform this trick. How about other classes of parasite?
Here we have a genus of barnacles, Sacculina, consisting of nine species, which take over the bodies of crabs and use them not to produce more crabs, but to produce more barnacles. Male crabs are even 'turned into' females by these parasites. The infected crabs go through the same motions they would perform when laying eggs to ensure their widest possible distribution, except that they don't lay eggs; they 'lay' barnacle lava.
Now, I'm not going to ask the obvious question here about why any compassionate intelligent designer would go to the trouble of creating these parasites which seem to serve no useful purpose other than to create more copies of themselves, and which give nothing back to the hosts they mercilessly parasitise. That would be too easy a point to score.
What I'm going to speculate on is whether another form of parasite can take over its hosts brain and use it not for the benefit of the host but for the benefit of the parasite. Could it happen to us? Remember, when considering organisms like parasites we are thinking about collections of replicators called genes which act together to build a machine for replicating themselves. This machine is the object we think of as an organism.
But why should this principle be confined to the objects produced by genes to replicate themselves? Why should it not also apply to other replicators like the memes which build cultures? Memes are units of cultural inheritance just as genes are units of genetic inheritance. Our cultures are meme machines built by memes to replicate memes. Through a combination of meme-gene coevolution we have arrived at where we are today - a cultural, civilised, ape with culturally evolved ethics and morals which enable us to work cooperatively together, at least within our local grouping.
What if a parasite could evolve the ability to take over this meme machine to produce more copies of itself by controlling the behaviour, beliefs, attitudes and even the ethics and morality of its hosts, not for the benefit of its hosts but for its own selfish ends? Indeed, in a Darwinian competitive environment where the only relevant test of fitness is the ability to produce the most copies, how else could such a parasite evolve? It is bound to evolve in a way which makes it better at controlling its host.
Apart from the possibility that the parasite has taken over even the rational thought processes of its victims, making them distrust evidence and even being afraid to consider it, it's hard to see why we don't regard religion as an example of just such a parasite. A combination of fear of a watching invisible thug and the hope of life after death for those afraid of it, seems to have done the trick. Protection from a non-existent threat and a promise which can never be delivered. Some reward for using our body and mind and taking away our freedom and independence, eh?
Yes, it could happen to us and very probably has.
The question now is whether we can rid ourselves of this malignant parasite and protect our children against infection by it. Probably the hardest part will be convincing its victims that they have been infected because, to them, just like the ants and crabs, it must feel entirely normal, otherwise the parasite wouldn't be in control.