Often marker for systemic disease
Most common in women 30-50 years of age
1:6 male to female
Usually a self-limited disease, resolving in 3-6 wks
1:6 male to female
Most often idiopathic
Drug induced (OCPs, sulfonamides, PCN)
Tuberculosis
Fungal disease (Coccidioidomycosis, Histoplasmosis)
Sarcoidosis
Ulcerative colitis
Crohn's disease
Behcet's disease
Cat scratch disease
Mononucleosis
Streptococcal pharyngitis (most common identifiable cause)
Normal pregnancy
Malignancy (leukemia, lymphoma)
Erythema nodosum
Erysipelas
Cutaneous larva migrans
Erythema induratum 
Cercarial dermatitis
Familial Mediterranean Fever
Bullosis diabeticorum
Insect bites