CD4 and infection: The principal conclusions from our study are that: (i) in a virus-infected animal, both naïve and memory CD4+ T cells show a similar and extended delay of ∼72 hours before they begin to divide; (ii) this is true in both lymphoid and non-lymphoid tissues; and (iii) this in vivo delay occurs despite viral antigen reaching T cell-stimulatory levels within 6–12 hours of infection.