Argument from ignorance

Argument from ignorance (from Latin: argumentum ad ignorantiam), also known as appeal to ignorance (in which ignorance represents 'a lack of contrary evidence'), is a fallacy in informal logic. It asserts that a proposition is true because it has not yet been proven false (or vice versa).

Speaker A: Infectious diseases are caused by microbes.
Speaker B: What a ridiculous idea!
Speaker A: How so?
Speaker B: It's obviously ridiculous.

Speaker B gives no evidence or reasoning, and when pressed, claims that Speaker A's statement is inherently absurd, thus applying the fallacy..