Serph has never seen the stone disease in action. Does it spread once its taken hold? He doesn't know. Fred's friend had told him that they weren't even sure if people were alive or dead once they had turned to stone...

"The form I fight with is a demon. The Atma virus is what changed me into that monster." Serph lifts his hand to the brand on his left cheek. "This mark is from the virus. I had it before arriving in this world."

So he had been a monster long before gaining his wings.