It's obvious Spike's been stung but Jack doesn't know the stings have any effect other than normal venom, which he assumes probably isn't so effective on an undead creature anyway. So it's a moment, as he continues to slash at the wasps, before he realises that Spike has turned and is staring at him. He glances round quickly, turns back to stab an approaching wasp, and then looks round again, puzzled and alarmed by the way Spike's staring at him.
"What?" he demands, and then ouch, he knows he's been stung too. "Come on," he urges, grabbing Spike's arm to at least draw him back againt the wall, so he'll have something at his back even if it's not Spike.
An odd, unexpected giggle bubbles up inside him, rather freaking him out.