It took several minutes’ soaking and gently easing the cloth away but eventually it fell clear and they could see what they were faced with.
‘Oh my god,’ said Horace quietly. The horror in his voice was obvious. Will made an inarticulate sound in his throat and, for a moment, turned his eyes away from the terrible sight of Halt’s arm.
The graze itself, which he might have expected to have dried and scabbed over by now, was still weeping. The flesh around it was coated with a discoloured mass of oozing, vile fluid. The rotting smell that Will had noticed earlier was now all too evident. Both young men instinctively recoiled from it. But perhaps worst of all was the flesh of the rest of the arm. It was swollen to almost half again its normal size. No wonder Halt had been rubbing and scratching at it for the past day, Will thought. And the entire swollen forearm was discoloured. A sickly yellow around the wound gradually gave way to a dark blue tone, shot with bands of livid red. He touched Halt’s arm gently with one forefinger. The skin was hot to the touch.
‘How did this happen? You cleaned and dressed the wound almost immediately!’ Horace said in a shocked, low voice. Both he and Will had seen their share of battles and their share of wounds in the past few years. Neither of them had ever seen anything like this. Neither of them had seen such a level of infection, for that was what this surely was, develop in a clean wound in such a short time.
Will’s face was grim as he studied the wound. Halt stirred fretfully, groaning and trying to reach with his other hand for the dreadful, discoloured arm. Will stopped him gently, forcing Halt’s free hand back down by his side.
‘There must have been something on the crossbow bolt,’ he said finally and Horace looked at him, not comprehending.
‘Something?’
‘Poison,’ Will said briefly.

Halt's Peril, Chapter 25