PDCD1 and infection: PD-1 is also upregulated during infection by the human immunodeficiency virus-1 (HIV-1) (Day et al., 2006) and hepatitis C virus (Evans et al., 2008) and in monkeys infected with the simian immunodeficiency virus (SIV) (Velu et al., 2009) and correlates with increased viral load (Barber et al., 2006, Blattman et al., 2009, Day et al., 2006, Palmer et al., 2013).