The cause of granuloma annulare is unknown, although there is much evidence that it is linked to the immune system. It has been reported to follow insect bites; sun exposure; tuberculin skin tests, ingestion of allopurinol; trauma; and viral infections, including Epstein-Barr, HIV, hepatitis C, and herpes zoster. Occasionally, granuloma annulare may be associated with diabetes or thyroid disease. 
What causes Granuloma annulare ?