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In Season 9, episode 6 Heaven Can‘t Wait, when Cas freaks out over the baby he‘s supposed to babysit running a fever. He says he‘s going to bring it to the hospital because he doesn‘t know what to do. But at the end of the episode, after Dean showed up, Cas had given it medication and admitted to overreacting earlier.
Cas wouldn‘t know what to do against a fever. He hadn‘t been a human for that long yet. Dean does. Because when he was a kid, he had to look out for Sam. He gave him medication when he was sick and no one else would.