CD4 and infection: Shimizu et al. observed a remarkable decrease in CD4+ T cells after 3 days infection with PRRSV [31]; and this study also reported slight decreases in CD8+ T cells at 3 dpi, followed by substantially increased levels [31], while at the same time, the ratios of CD4+/CD8+ T cells were significantly lower between day 3 and 28 post-inoculation compared with day 0 [31].