Not at all. We say "That's just sci-fi" when a story is written about some kind of effect that is extraordinary and without a basis in known science or technology.

A pandemic is perfectly plausible.

Everything is plausible with the benefit of hindsight. E.g. "Tintin on the Moon" predated the Moon landings. From the perspective of e.g. 1850, the idea of landing on the Moon was "extraordinary and without a basis in known science or technology".