CD4 and infection: A mode of retroviral infection of CD4+ T cells that is more efficient than cell-free spread is cell-to-cell spread (Dale et al., 2013, Sattentau, 2008), exemplified by virological synapses (VSs) and associated structures that drive efficient high-multiplicity infection in vitro (Dale et al., 2013, Sattentau, 2008) and may dominate viral dissemination in vivo (Murooka et al., 2012, Sewald et al., 2012).