MArcomage

Cute, as is the alternative you mention.

(The danger might be, though, that it mainly slows down and isn't fun, like the holy effect put's useless cards in the other hand...)

That might be true, just trying to come upp with something that counters stacking lots of a single keyword as it currently is really really strong.

Hmmm... I think it should be Quick, because if you play it (and discard two of enemy's cards, the enemy is able to discard 4 of your cards, and so the virus "spreads" more)

Then it would be a pandemic, not a virus. I think it's better as it is now.
I like to play Arcomage, not catch the Virus.

I would prefer that this card only replaces common Card since this Card is only a common and with this more than easy to gain.

I agree, this card should be Quick, Zero cost and should only replace opponent's common and uncommon cards if they cost more than 20 resources in total (thus it's still annoying but doesn't cripple the opponent completely and ruining the game)

But overall it's not a very useful card, since the Cursed Amulet card does the exact same thing, only it's not random and it's quite annoying as well.

this is very different form cursed amulet but i do realise how annoying it would be if implented, but maybe something in the lines of discarding only if there is more than 3 cards with the same keyword in oponents hand

I don't think it should be keyword-dependent. If someone wants to take this into his deck, he should be prepared to have it used against him as well and just as easily. Limiting it to Common cards seems reasonable, however. Also, I would hope that it would be just as likely to discard another of itself rather than focusing first on hand positions that haven't already been affected.

I know that this isn't implementable, but it would be neat if hand positions could become "immune" after being infected once, preventing any further Virus discards to that specific position. (If such a limitation wanted to be added, an "immunity" could perhaps be conveyed by already having Virus on hand. "No effect if opponent has Virus in hand.")