Okay.. this is hard to explain, so bear with me. It happened in the Trainer.
Judging by this picture, it looks like Moreric has cast Aflatoxin on something of mine. It was a Virus that infected his Mummy, but it was only infected by one of his Viruses. Here's what happened a couple turns back:
He cast Aflatoxin on my Virus that was in the first creature slot (middle row, middle slot).[/li]I sacrificed the Virus to infect a Mummy of his. I got no Malignant Cell.[/li]Couple turns later, he plays a Virus. I play a Virus on my turn. I have no other creatures out, so it ends up on the same slot my first Virus was on; the first slot.[/li]He infects my Virus with his Virus on his turn.[/li]I infect his Mummy on my turn and play 2 Dragons. I get a Cell from the sacrificed, infected Virus.[/li]
Have fun recreating this. x)
