When a player visits the dungeon and reveals a Disease card from their hand they suffer a -1 Attack penalty for each Disease revealed. This penalty can affect Magic Attack as well if it is an available choice.
Diseases are most often obtained when defeating certain monsters, and typically removed from the player's deck through resting on their turn, or visiting the dungeon with a cleric hero.
When a Disease is destroyed it is placed back into the stack of Disease cards. It is not permanently removed from the game, just put back into place for any other unlucky player to obtain later.
If there are not enough Disease cards, take any card not being used presently and treat it as a Disease card.