Macrophage activation syndrome (MAS) is a disorder whereby the immune system generates very high levels of substances (cytokines) that promote inflammation to the extent dysfunction occurs in multiple organ systems which if unchecked, is frequently fatal to the affected individual. This can occur in the setting of a number of different immune system disorders including, systemic lupus, systemic-onset juvenile arthritis, and adult-onset Still's disease. MAS can also occur in response to infection with certain viruses such as Epstein-Barr virus (EBV) and malignancies involving lymphocytes.