CD4 and infection: First, for times small compared to the CD4 down-modulation timescale, i.e., t ≪ td, and when changes in viral and cell numbers are small, we find thatSecond, for times large compared to the time required for viral load evolution to reach pseudo steady state, i.e., t ≫ teq, we obtainwhere we define k1 as the mean rate of the second infection of singly infected cells.