Broken armour. Blood and viscera leaks from their body.
If the NPC crits with a melee attack, have the character on the receiving end of the bite make an upgraded hard ( :eD: :eD: :eC: )Resilience check. If the character fails, they are infected. Finding an antidote is of the utmost importance.
The time a character has to find an antidote, should be narratively appropriate, as the effects stated below don't take effect until the infection has taken hold. An infected NPC reduces their Intellect and Cunning by 1 to a minimum of 1; and increase Brawn by 1 (not affecting soak/wound threshold) Wound threshold may be increased by 1 to 5 at GM's discretion.
Player characters are special, however. A cut above the rest. Should a PC get infected, and an antidote is not found in time, apply the above changes, save Intellect, as the player characters maintain their sense of self through the infection. The infected PC still craves the flesh of the living, and must make :eD: :eC: Resilience, or Discipline checks at narratively appropriate intervals, as determined by the GM to resist the urge to eat their teammates.
Application of the antidote will return PC's to their pre-infection status. Infected NPC's given an antidote will have a few moment's clarity before succumbing to death.