Combined with our experimental findings that MDV/RB1B infection induced a higher level of IL-6 mRNA in the spleen at 14, 21, and 28 dpi than MDV/CVI988, we speculated that the persistently high levels of IL-6 expression in lymphoid organs might be a differential marker between vvMDV strain infection and MDV/CVI988 immunization. Here, IL6 is linked to infection.