It often seems that the Kafka Pandemic is a lesson in social psychology that 100 doctorates stacked on top of one another could never match. When you experience it, your view of human reasoning and behavior is forever changed.
No matter your knowledge of the scarcity of thought in humans, it is never enough. Reflective interlocutors reduce themselves to stimulus-response machines, intelligent ones to a morass of non-sequitur prejudice. Jaw-dropping is inadequate as a description.
Here is an approach that will work occasionally. Rationality might not break through, but it might nag them. Make them feel uncomfortable. Make them pause before trying again. Even if only because they know they won't get away with it next time.
Whenever you hear an irrational statement about the disease in question, say, "Would you say that about AIDS?"
The idea is to lure them to where you have rhetorical parity. If the answer is yes, you are on solid ground. If the answer is no, you are on solid ground. If the person evades the question, you are on solid ground.