He feels compelled to snap a salute, but it isn't as clean as it would be with a commanding officer. There's some looseness to it, a little friendly sarcasm dusting the underlying respect.

"Thanks, doc," he says, turning neatly on his heel and heading for the sickbay entrance. McCoy doesn't need to tell him twice to get out of here. He'll be relieved to be back in quarantine after this, just so he has some space to think.