This phenomenon is sometimes called 'brain jacking'. The protozoa Toxiplasma gondii, in one part of it's life cycle infects the brains of rats, and it target's the centres of the brain that mediate the fear response. Rats have a natural aversion to the scent of cat urine which helps them to avoid one of their main predators. However once infected with Toxoplasm gondii the rat can not only lose it's fear of the scent of cat urine, but it can make the scent appear attractive. This dramatically increases the chance of the rat being caught and eaten by a cat.
And the second part of Toxoplasm gondii's life cycle is in the cells of cats. Acanthocephalans are a phylum of parasitic worms which like the Toxiplasma have a complex life cycle. Expelled in the faeces of the primary host, usually ducks, embrionic worms are ingested by the secondary host, usually a crustacean such as Gammarus lacustris. Now these crustaceans avoid predators, the same ducks, by keeping out of the light, and avoiding the surface of the water.
However, infested with Acanthocephalans their behaviour changes to the opposite, the crustaceans become attracted to light, and seek out the surface of the water, where they become easy prey for the ducks. Having been eaten by a duck the parasite can complete it's life cycle and produce more embryos to be expelled in the duck faeces.
One of the more dramatic examples is a fungus, Ophiocordyceps unilateralis, which infects carpenter ants. Ant's spend most of their time in the tree top, but once infected with spores of the fungus it is compelled to travel down to the forest floor to find conditions which better suit the fungus, typically about 25cm off the ground, on the north side of the tree. Once there they bite into a leaf, lock their jaws, and wait while the fungus liquefies their internal organs, except the muscles in the jaws, and turns the ant into food.
A Different Kind of ParasiteNow imagine another kind of parasite. An economic parasite, a parasite that changes our behaviour. This parasite encourages risky behaviour in consumers, and in particular it induces them to take on loans they can't afford. The parasite feeds on interest payments, so the more interest it can gather the larger it can grow. But the parasite also disables the economy's natural defences by co-opting the parts of the economy concerned with regulation of finance so as to remove controls on lending behaviour. The parasite causes the heads of government to believe that a benefit for the parasite is a benefit for the host economy and if they give the parasitic organisms free range it will be for the greater good.
Members of the parasitic community take up advisory positions in government where they help to formulate policies more conducive to parasitic activity. But the parasites also attack the host government with "lobbyists" which act like viruses, they have no life of their own, but exist solely to insinuate parasite friendly memes into government and public discourse. It goes much deeper than this.
The parasite has a complex life cycle, and part of it is spent in educational institutions where it infects those who study economics - it changes their cognitive maps and their post-graduate behaviour. Thy become vectors of the disease.
One of the cognitive tricks the parasite does, especially in the educational part of its life cycle, is to make the commentators blind to debt, and deaf to those who can see debt. This means that the parasite can cause massive accumulations of debt that never occur on the radar of economic regulators. Now the host economy can only sustain so much debt, and accumulations of debt suppress demand, causing slow downs or recessions. Once a recession is in play, the parasite goes into a dormant phase during which time the economy can recover and regain strength, and allow debt levels to reduce to non-pathological levels. From time to time debt accumulates to such an extent that it causes a major collapse of economic activity which threatens both host and parasite. This infection is epidemic in the world's economies, and some have fatally succumbed to it. Even China has now caught the infection. Removing the infection is going to involve a coordinated systemic approach.