Poisoning the well is a logical fallacy that uses the association of negative emotions to distract a subject from actual evidence in an argument.  Usually, the arguer attempts to present any information that could produce a biased opinion of the subject, either in support or against it.  

The poisoning the well fallacy is related to the ad hominem argument, in that the subject is attacked or defended based on some irrelevant quality, rather than the actual evidence.

Example of poisoning the well

Big Pharma is immoral and greedy. They hide evidence of the failure of vaccines. They bribe the FDA, the CDC, and WHO. Therefore, vaccines shouldn’t be trusted.