Generally caused by bacterial infection (necrosis of the skin) (especially with Columnaris disease) (Ill. 32+36+38+39+41+43)
but in some cases by parasites (Pleistophora) under the skin in the muscle tissue (Neon disease) (Ill. 36),
which is easily mistaken for False Neon disease (Ill. 36).
Also parasites on the skin (Tetrahymena, Piscinoodinium) may cause white patches (Ill. 34+40).
Some infections may occur simultaneously (e.g., bacterial and fungal infections). (Ill. 45).
Sometimes the white patch appears in a specific place, e.g., on the mouth (Ill. 43)
or head but in a later stage it may spread to the whole body (Ill. 43)
or change into some sort of crater. (Ill. 33).
Tip: Infections on the head or skin can be quite similar, confusing and caused by different pathogens.