Plague is an artifact that is used on any player, before destiny is drawn, to force him or her to lose three ships to the warp and discard one card of each type (negotiate, attack, morph, artifact, flare, etc).
Note that "encounter" is not a card type itself, but a category of types. Thus, if a player has a Plague played on him or her and his or her hand contains 1012 N N , just discarding a N will not be enough; the player must discard the 10 or 12 as well to satisfy the requirement of "one card of each type."