The only virus ever created by Game Freak was Pokérus, so I don't think an embodiment of a disease would work well. It's just too tiny, nor even multicellular. Pokemon that use diseases as defense-mechanismm could be cool, but would you need a new type for that (à la Snakewood)? A mosquito pokemon would be a first (plus part above), and of course a rodent with, for example, Poison Touch or an ability to spread Pokérus would be cool. We don't have mons based on salmon, haddock, tuna...also, are there any chicken-inspired pokes? Oh yeah, and a mon lookalike to Sunflora, but with a Mega Stone, based on Flowey would be baller (and traumatizing).