[s11e1] Out Of The Darkness, Into The Fire Now

"It’s a localized infection," Sam whispered, hopping out of the car to inspect a fallen road worker. "Whatever that smoke was, it changed them."

In the quiet of a darkened hallway, Dean looked down at the baby. He felt a pull, a strange, magnetic tether he couldn't explain. He moved the infant’s blanket aside, and his heart stopped. There, on the baby's tiny shoulder, was the mark. The Mark of Cain. [S11E1] Out of the Darkness, Into the Fire

If you want to explore more about this specific era of the show: The psychological bond between Sam’s secret struggle with the Rabid infection Castiel’s torture at the hands of the angels "It’s a localized infection," Sam whispered, hopping out

Sam didn't answer immediately. He was staring at the road ahead, where the asphalt seemed to shimmer with heat that shouldn't be there. "We let it out, Dean. We actually let it out." He moved the infant’s blanket aside, and his heart stopped

The town was a graveyard of the living dead. Amidst the chaos, they found a young deputy, Jenna Olson, terrified and clutching a shotgun in the local hospital. She was protecting a newborn baby—a girl whose mother had succumbed to the darkness moments after birth.

The Darkness wasn't just a force of nature. It wasn't just a cloud of smoke. It was a person. And he was holding her in his arms.