• Immune reaction involving T helper type 1 and interferon-γ pathways

Precipitating factors:

1. Infections (~90% of cases)
   • Viral
     • HSV 1 (50%)
     • HSV 2
     • VZV/EBV/CMV
     • HHV/HIV
     • others
   • Bacterial
     • Mycoplasma pneumoniae
     • “Mycoplasma mucositis” Chlamydophila
     • Salmonella
     • Mycobacterium tuberculosis
   • Fungal (rare)
     • Histoplasma capsulatum
     • Dermatophytes
2. Drugs (rare)
   1. NSAIDS
   2. Sulfonamides
   3. Anticonvulsants
   4. Allopurinol
   5. Other antibiotics
3. Exposures (rare)
   1. Poison ivy
4. Systemic diseases (rare)
   1. Inflammatory bowel disease
   2. Lupus erythematosus (Rowell syndrome)
   3. Behçet disease

Triggers for outbreaks:

• Trauma
• Cold
• UV and orthovoltage irradiation