A stonefish or other sting is a possibility. I just remembered the bit about EXTREME PAIN. While a very painful infection would indicate removal from the game, not necessarily so with a sting.

A stonefish sting is one of the most painful stings in the world (they say). Rapid pulse is one of the symptoms. However, there is an anti-venom that can be used to treat a sting, which Survivor medics should have on hand.

Other stinging critters in Samoa are the man of war, cone shells (paralytics). Coral cuts can be quite painful and serious. Lionfish and scorpionfish are related to stonefish, but more often encountered by divers. The stonefish cling to coral outcrops and hide buried in sand ... people step on them, and you are not supposed to go barefoot in the ocean where there are stonefish. (Greg's shoes seen hanging in shelter.) Anyway, probably not it, but I'll post the info on it.

STONEFISH

The Stonefish is virtually invisible amongst rocks. The creature has venomous spines on its back that it erects when threatened or stepped on. This fish lives in the tropical regions, and as far south as Forster/Tuncurry in NSW.

SIGNS AND SYMPTOMS

severe pain at the site of envenomation
swelling
open wound and/or bleeding
irrational behaviour
rapid pulse
breathing difficulties
collapse - coma

CARE AND TREATMENT

Make a balance
call an ambulance
HOT water immersion of the site to relieve pain - ensure that the water will not scald the casualty
EAR and/or CPR as required