A 'Sacred Cow' is a term used for something too highly regarded to be open to criticism or curtailment. It is something that is considered to have a status that allows it to avoid any criticism or questioning. The phrase has arisen from and generally refers to the veneration of the cow in Hinduism (a religion).

People have been groomed, through lies and shelter from the truth about vaccines, to believe they are some miraculous protection against diseases. So much so that anyone who challenges that are viewed as some sort of social heretic.

This BGB reproduction of an original Dave Cullen video goes into this phenomenon in more depth.

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