CD4 and infection: Although CD8+ T cells were thought to be important to eliminate influenza viruses in mice [33] and preexisting influenza-specific CD4+ T cells correlated with disease severity in humans [34], the role of T cells, either CD8+ or CD4+ T cells, in human influenza virus infection remains elusive since both CD4+ and CD8+ T cells were expanded 7 days after challenge infection [34].