List some of the common underlying conditions associated with erythema nodosum.

Streptococcal infection of the upper respiratory tract and medications (especially oral contraceptives, sulfonamides, penicillins, bromides, and iodides) are among the most common known causes. Other triggers include deep fungal infections (such as coccidioidomycosis), Yersinia infection, pregnancy, sarcoidosis, inflammatory bowel disease, and leukemia. Many episodes are idiopathic: A cause is not identified in one third to one half of cases.