How dangerous is belief in conspiracy theories?

Dutch journalist Maarten Reijnders wrote a book about the currently most popular and (in)famous conspiracy theories and their proponents in the Netherlands. The book, titled Complotdenkers – Hoe gevaarlijk is het geloof in samenzweringstheorieën? (‘Conspiracists – How dangerous is belief in conspiracy theories?’) was deliberately published on 11 September 2016, because the 9/11 Truth movement is one of the most prominent of these phenomena in Western society at the moment. Skeptic Pepijn van Erp wrote a review; here is an excerpt:

Reijnders defines ‘conspiracists’ as people who believe in lots of different conspiracy theories at the same time, or draw rather far-reaching conclusions from such a conspiracy belief system. He calls a collective of such conspiracists a conspiracy church. That is a broad church, with many schisms. With liberals and literalists. With soft, kind and harmless believers, but also with some extremist fundamentalists. (…)

It can lead to contempt for innocent people and minorities, and we can still see enough suffering caused by that today. And we also know the example of the disastrous HIV/AIDS policy in South Africa under Mbeki, based on completely pseudoscientific ideas, that has led to an early death for an estimated 330,000 people.