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The human illnesses are the worst part of losing his grace. The low level ebb of the grace is a constant, grating pain, but when Castiel gets knocked down by the flu, forced off the road on his way back to the bunker after weeks of being away, tracking down leads… that is the worst. Not just the physical discomfort, but being kept from companionship and the only place on Earth that reminds him of Dean.
He's still a full day's drive from the bunker and coughing up a lung, as the saying goes, when Sam calls. A lead, a real one, it sounds like, and Sam is asking for his help, which hasn't happened since Castiel couldn't fight off his one demon to help Sam with the pair on him. The dislocated shoulder and hospital visit that had been the result are still fresh enough that Sam has been leery of teaming up with Castiel.
This time it's not Sam's injuries that keep them from teaming up, but Castiel's inability to fight off the influenza virus. He is so weak that a virus can remove him from play. It is disheartening.
So Sam pretends it's not a lead and Castiel lets him, pulling the sheet up to his neck and letting the coughing overtake him, closing his eyes and imagining being back in the bunker, warm and safe, and even, after a moment of hesitation, Dean and Sam both being there, happy and healthy and whole.