"Almost got me with a weaponised meme."
  - from The Quantum Thief
In nature, there are a number of brain parasites that control their hosts' behavior, turning them partially or fully into zombies. Many of these are fatal. Humans are mostly safe from these threats, but I believe that we face an analogous threat from memetic "brain worms" - ideas that crawl inside our heads, hull out a little portion of our brains, and begin steering our thoughts toward intellectual self-destruction. For example, there's "Austrianism," which convinces people that loose monetary policy is the source of all macroeconomic problems. But that is hardly the worst.
Perhaps the most potent and deadly memetic parasite I know of is called "Race and IQ". This is the belief that average differences in measured IQ across different consensus-defined racial groups are really, really important to society. 
That's the core belief, but there are some associated ones. 
The first is a persecution complex. "Race and IQ" people believe that they are oppressed rationalists/empiricists fighting the good fight against mainstream/liberal culture, which is engaged in a massive effort to deny or cover up The Truth. This can cause severe perceptual distortions, e.g. the notion that people who deny that race is an important or interesting correlate of IQ are denying the validity or existence of IQ itself, or the partial heredity of cognitive abilities. Like most persecution complexes, it also tends to cause aggressive, paranoid behavior.
The second is the certainty that the aforementioned average IQ gaps are due entirely to deep, fundamental (but always unspecified or hypothesized) genetic differences between broad racial ancestries, rather than to things like selective immigration.
The third is the belief that "Race and IQ" is responsible for many or even most of the economic phenomena in the world - the wealth and poverty of nations, growth rates, savings rates, bubbles, etc.
Furthermore, "Race and IQ" seems to act as a sort of gateway drug. Once people's brains have been infected with this parasite, they become more susceptible to stereotypes of all kinds. Soon, they will believe every stereotype about women, about gays, about Jews, about "Latin lovers," about Germany and Greece - about anything and anyone. The older the stereotype, and the more associated with right-wing politics it is, the more likely they are to believe it. They will start inventing new stereotypes about population genetics to explain almost any phenomenon they see - they become addicted to stereotyping. This may be a side effect of the persecution complex - once you think that the mainstream is engaged in a big cover-up effort against one stereotype, why not all of them? And not only do the "Race and IQ" zombies believe in stereotypes, they start trying to apply them in daily life. You know a meme is a brain-eater when it makes people incapable of distinguishing between statistical significance and goodness-of-fit!
In fact, this brain parasite has been around for a long time. In The Great Gatsby, a character suddenly starts spouting the "Race and IQ" canon:

“Civilization’s going to pieces,” broke out Tom violently. “I’ve gotten to be a terrible pessimist about things. Have you read ‘The Rise of the Colored Empires’ by this man Goddard?” 

“Why, no,” I answered, rather surprised by his tone. 

“Well, it’s a fine book, and everybody ought to read it. The idea is if we don’t look out the white race will be — will be utterly submerged. It’s all scientific stuff; it’s been proved.” 

“Tom’s getting very profound,” said Daisy, with an expression of unthoughtful sadness. “He reads deep books with long words in them. What was that word we ——” 

“Well, these books are all scientific,” insisted Tom, glancing at her impatiently. “This fellow has worked out the whole thing. It’s up to us, who are the dominant race, to watch out or these other races will have control of things.”...

“This idea is that we’re Nordics. I am, and you are, and you are, and ——” After an infinitesimal hesitation he included Daisy with a slight nod, and she winked at me again. “— And we’ve produced all the things that go to make civilization — oh, science and art, and all that. Do you see?” 

There was something pathetic in his concentration, as if his complacency, more acute than of old, was not enough to him any more...Something was making him nibble at the edge of stale ideas as if his sturdy physical egotism no longer nourished his peremptory heart.

If this describes someone you've interacted with, or read on the internet, well, now you know what it is.
If you read blogs, you've probably seen a few "Race and IQ" zombies infesting comment sections. But the scary thing about this memetic parasite is that it has shown an ability to (occasionally) infect even the most intelligent minds. It ate James Watson's brain. It might or might not have eaten William Shockley's brain. It has probably eaten the brains of one or two economists over the years. It's sad and scary to see a once powerful mind reduced from exploring the mysteries of the Universe to exploring this lazy, tired, mostly-irrational worldview.

Why is the human brain so vulnerable to this parasite? Here are a few conjectures:

* There is innate pleasure in being the iconoclast - the one guy who's right while all of society is wrong. "Race and IQ", with its persecution complex and its veneer of science-y-ness, lets you play at being Giordano Bruno.

* When smart guys get old they usually lose their edge. Being smart has been part of their self-image all their lives, and now that's being taken away from them by nature. Imagining oneself as part of a naturally intellectually superior race is a way of substituting group pride for individual pride.

* Tribalism and racism, of course, are deeply rooted. People want a reason to believe they have some giant gang out there that will back them up. "Race and IQ" feeds this desire.

In any case, "Race and IQ" is a worrying parasite because I don't know of a cure. Memetic parasites cannot be cleansed with logic and facts, but often there is a sort of anti-parasite agent available - a benign alternative belief system that the infected can be diverted towards. Austrianism, for example, can be cured with MMT. But I don't know of any equivalent "halfway house" belief system that is effective in getting people off of the "Race and IQ" train.

I think finding a cure for this brain parasite would yield substantial rewards for public health.