smear the end of the punji stick with manure, either animal or human, that will ensure the wound gets infected, I think that's what the Viet Cong did.
A mixture of chicken, pig, and/or human feces mixed with an approximately equal amount of blood.
In fuedal Japan, the mixture is an equal amount of fresh chicken or pigeon feces, blood, and finely chopped-up raw shellfish. I assume that land snails and/or slugs could be chopped up and utilized in the same way.
This mixture was used on a kind of calthrop called a "tetsu-bishi". It was also painted on arrowheads, shaken ("throwing stars", although I detest the term), tanto knives, and so on.
Most of these weapons utilize a puncture-style blade. I can assume--given my medical training--that any deep puncture wound with this mixture will most likely bring about death in 3 to 5 days at the most unless a modern hospital with a full compliment of neurosurgeons and trauma specialists can be accessed very quickly right after the wound happens.
I would expect a combination of gas gangrene, necrosis, septicaemia, tetanus, wound botulism, and mental delerium to set in relatively quickly.