CCR5 and infection: It has been suggested that this relates to the abundance of CCR5-expressing macrophages at sites of primary infection, but an alternative explanation suggests that host cell division rates may determine the dominant viral type.[17], [18] According to this model, CCR5-expressing cells, predominantly memory/activated T-cells have higher rates of turnover,[19], [20] and thus produce more of the virus which preferentially infects them, in this case R5-tropic virus.