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One reason to do that kind of crappy trades (offer a zubat and ask for a legendary) if you have 2 copies of the game but only one 3DS, so you can offer a crappy Pokemon in your objective cartride and then go to the GTS with your legendary and look for a zubat and change it for your legendary...
Hi regarding pokerus, if its still infected (if says PKRUS in the stats screen) it can pass the pokerus to others Pokemon in the team after a battle (it's a bit random but should take long)
But at least in X/Y at midnight the Pokemon heal the pokerus and it can pass the condition any more to...