Black fungus is caused by a fungus named mucor, found on wet surfaces.
Due to a few medicines used in covid patients such as life-saving dexamethasone, prednisolone, methylprednisolone, dexona, etc, the compounds in them suppress the immune system, causing fungus attacks in a diabetic patient.
Talking about the symptoms of black fungus, it appears three days after recovering from the Covid-19 infection.