This narration المطعون شهيد is found in Sunni books of Hadeeth ( Riyadh al-Saliheen 1:1353 from Abu Huraira) but I did not find it in our authentic books of Hadeeth. It has been mentioned in few of our books of Fiqh but the meaning of مطعون is not a victim of plague but a stabbed person. مطعون can be from طاعون plague and can be from طعن which means stabbing. The meaning of the narration if it was authentic can be a victim of plague or a stabbed to death.
If it is an authentic, it does not mean a real martyr who should not be given Ghusl, but it means the reward which he gets because he was a victim.