Dean was getting ready to rip Cas a new one for disappearing on him again when the angel mentioned Jo was possessed.
"No she wasn't," Dean said quickly. "Not possible."
Whatever Jo was, whatever flaws she had as a hunter, that wasn't one. Jo carried an anti-possession charm on her at all times. Ellen had seen to that. It's what made it so clear when War was in town that something was up. Jo couldn't be possessed. If the thing wasn't outright around her neck, it was in a pocket. And if it wasn't, something was really, really wrong. Jo was not stupid enough to go walking--and hunting--through the world with a vacancy sign hanging around her neck.
Hunters were always possession targets.
"Bit?"
Dean stood up and moved to where Harry stood to take a look.
"I'm fucking serious, Cas. Jo and Ellen always have anti-possession charms on 'em. Gimme a sec, I'll check her pockets."
He was up in another second, a half-smile on his face. First-aid for the Winchesters was a little different. It was bandaids, gauze, thread, a needle, tweezers, and a bottle of whiskey. He disappeared into the bathroom for a second and came back with a wet wash cloth for Jo's ankle, and the bottle of whiskey in the other hand.
Disinfectant.
Castiel looked evenly at Dean. "Which is the reason I brought her to you. You should watch her and see if there is anything else amiss. What was within her reacted as a demon."
Harry scowled at that. "Great. More freaking demons." He took the tools from Dean and began to clean the area around the bite wound. He was willing to do nurse work and let Dean talk to the angel, but Dean could sew up his own girlfriend, if it came to that. The bite didn't look fresh, but it wasn't very deep. The whiskey had a solid aroma, and Harry took a quick swallow of it before using it on the wound.
Reacted as a demon? That didn't make Dean too happy. Two things were possible, here, and neither was fun.
One: Jo HAD been possessed. And she'd lost her anti-possession charm, leaving her wide open for it to happen again. Two: She had the charm, and had it the entire time. And whatever happened just looked like possession to Cas. If it was good enough to fool an angel, at least on the surface, Dean didn't want to think about what was actually going on.
There was nothing around her neck or wrists. He'd check her pockets once Harry was done taking a look at the bite.
He nodded with his head toward Jo's ankle, and where Harry was sitting. From here, the bite didn't look human. He'd have to see it without blood still clinging to to it to make his own judgement on what bit her. "What do you think?" he asked Harry.