I think it originated with pooping on swords/polearms: you're a peasant, just before the battle, and you have a massive adrenaline dump, part of which involves clearing out the intestines, so if you manage to channel your inner cat, as you barely get your breeches off you stick your weapon underneath. Then you tell all your buddies you totally just did that on purpose, to cause sepsis among the enemy...

If you're an grizzled noncom equivalent, when the newbies suddenly wind up with a fresh pile of squishy goop, you tell them the same story, and have them run their weapons through it, making them feel better about having squatted down in front of their buddies?

Lagniappe: how about this as a way to take physiological signs of adrenaline and turn them into an "I meant to do that" moment? https://www.youtube.com/watch?v=u_rotEpFtZU&t=64s

It takes the opponent out of the next battle which in a long campaign might improve your chances of getting home alive

It mobilises a number of resources to patch up a wounded/sick soldier.

There is a myth that the 5.56 round was designed to wound and not to kill (contrary to the 7.62 round the soviet side used) as it would make war costly in a very visible manner.

I guess the idea that inducing "Sepsis by poop" follows the same line of thinking.