Searching these forums and reading several topics on how to devise a healthy economy, I found out that the most basic concepts in a MMO's economy are the money faucet and the money sink. That means in order to allow the game self-regulate there must be a signal between faucet and sink. As players open the faucet, the game should activate a mechanism that opens the sink's hole wider to deflate the economy.
Then I recalled watching a documentary about the Black Death, specifically this segment is very interesting:
http://www.youtube.com/watch?feature=player_detailpage&v=TczFkRnBto8#t=179s
But of course the game can't kill its most valuable asset, players. Instead plages will target items. As players harvest raw materials, trade or produce artifacts, and loot, their inventories will randomly harbor these "bugs" that devour tangible values of certain nature. For example, moths could swallow fabrics, termites will eat wooden objects, and corrosion will make metals (and maybe even gold coins, thru alchemic corrosion) to disappear from the World. Basically in my design items will have infinite lifespan, as long as they aren't destroyed by a pest.
I think this is also a good way to keep players from hoarding insane amounts of artifacts. There may be no limit to what you can store or the weight you can carry in your inventory. But if you own a huge stack of trash, it's pretty possible that bugs are happily living and reproducing in the heart of that heap. Since most plages will be always present, players could produce or buy remedies. These will be use-once items that remove one sample of a pest at a time, and the effectivity will be divided by the size of the inventory, and is reduced in time. In example, each rat eats only a portion of poison, and you will need more portions to put in all their entrances, but each will spoil eventually and must be replaced.
Finally I guess the variety in plages and the World's dynamics will also reduce the chances of over-production of "pesticides". Hope "mothballs" won't be the favorite item on top of the "mega-sword of all might".
So what do you people think of this mechanic? Thanks in advance for your comments!