This can apply to both parasitic and candida infections.
I have isolated a specimen of my parasites, and added india ink to resolve some issues dealing with microscopic observation.
The result was a discovery of small pockets of lively activity, like little ant colonies. Wasn't sure what these were, until I found a massive patch of very angry looking bugs! Actually these are too small to even be bacteria.
Did some research, and decided that this phenomenon is host immuno defense proteins.
You see, when the mucus membranes are under attack, they produce cell penetrating hydrophobic proteins to kill bacteria and other pathogens or parasites.
India ink bonds with hydrophobic proteins, and therefore I conclude this is the case.
I have included a video of the phenomenon, so you can see how active these proteins are.
When you poo, these proteins will line the sensitive tissue of the anus, and cause irritation resulting in itch.
There you are folks, this is the explanation never given, but is now finally elucidated.
I would assume the same sort of thing can apply to mucus tissue of other areas of the body like the vagina for example.
Here is the footage: