Erythema nodosum (EN) is the most common type of septal panniculitis (inflammation of the subcutaneous fat) and results from a delayed hypersensitivity to various stimuli, such as bacterial and viral infections (most commonly Streptococcus and Epstein-Barr virus [EBV], respectively), medications, malignancies, inflammatory bowel disease (see Crohn disease, ulcerative colitis), fungal infections, and collagen vascular diseases. However, in 33%-50% of cases, no associations are found. A detailed list of etiologies is below.

The eruption persists for 3-6 weeks and spontaneously regresses without ulceration, scarring, or atrophy.

EN occurs in boys and girls in equal proportions before puberty (it rarely occurs before age 2); however, after puberty, females are more frequently affected, as in the adult population. Recurrences with reappearance of the precipitating factor(s) are reported.

More commonly associated infections:

• Streptococcus infection, especially pharyngitis
• Mycoplasma
• Upper respiratory viruses (eg, EBV)
• Mycobacteria (tuberculosis [TB] and atypical)
• Coccidioidomycosis in endemic areas

Less common infectious associations:

• Yersinia, Shigella, Campylobacter, Salmonella sp gastroenteritis
• Cat-scratch disease, Chlamydia, syphilis, pertussis, leprosy, and numerous other bacterial infections
• Hepatitis B, HIV
• Blastomycosis, histoplasmosis, sporotrichosis, Giardia

Inflammatory etiologies:

• Sarcoidosis
• Inflammatory bowel disease 
• Behçet disease

Malignancy (especially leukemia)

Drugs:

• Oral contraceptives, sulfonamides, penicillins, cephalosporins, macrolide antibiotics

Pregnancy