Let’s define the Ultimate Plague as the deadliest plague imaginable. You can no doubt imagine some terrible bacterium that can kill the entire population of the earth, humans and animals, in a matter of hours. Now, if two diseases are identical except that one is imaginary and the other is real, then clearly the imaginary one cannot be the Ultimate Plague, because imaginary diseases aren’t deadly; only real diseases are deadly. Therefore, the Ultimate Plague is real. QED.
That this argument is a load of bollocks is made clear by the fact that we’re still alive. But it follows the same logic as Thomas Aquinas’s ontological argument for the existence of God, so therefore that argument must be a load of bollocks as well.