I couldn't miss the chance to practice my drawing!
Virus was an ability available to the Arcanist class at level 12, which reduced the target's strength and dexterity by 15% for ten seconds. The trait "Supervirus" also enabled it to reduce intelligence and mind. A condition called "Antibody" would be applied when it wore off, preventing Virus from afflicting the target again for one minute. The action was available as a cross-class action and could only be used by Conjurers, Thaumaturge, Arcanist, White Mages, Black Mages, Summoners, and Scholars.
Due to the development team's desires to remove the cross-class system and unneeded actions for the Stormblood expansion, Virus and the Supervirus trait were both removed entirely. The ability was instead replaced with the caster cross-role action Addle.