Model A, illustrated in Fig. 4, assumes that CD163 is essential for productive PRRSV infection, but that shedding of the receptor or the mAB binding domain within the 18 h infection period may be responsible for the high prevalence of infected CD163 negative cells that was observed 18 hpi in some samples (Figs. 2d and 3c). The gene discussed is CD163; the disease is infection.