BLU can already inflict a fairly potent plague with the spell Lowing, though probably with limited accuracy (I don't know I don't play BLU). I don't really see Virus as a white mage kind of spell, plus WHM already has Auspice to reduce TP gain. I can see them giving it to BLM and/or SCH but that wouldn't be very fair to RDM, which are (apparently) supposed to be the masters of enfeebling and yet have so few exclusive enfeebling spells.