What fungus causes mycosis fungoides?

None! Mycosis fungoides is the common name for the most common type of cutaneous T-cell lymphoma. A lymphoma is a malignancy of the white blood cells that are known as lymphocytes. Mycosis fungoides primary involves the skin and produces patches, plaques and tumors. The patches may be scaly or flaky and can thus look a lot like an ordinary rash such as the rash of proriasis or of a non-specific dermatitis. Many times, the plaque-like rash will look like a fungal dermatitis, and thus the name for the syndrome. However, there really is not a fungus involved.