CD4 and infection: In addition, there is a high prevalence of comorbid conditions and confounding factors that may interfere with the effects of HIV on the CNS (Weiss et al., 2010), such as CD4 nadir count (Munoz-Moreno et al., 2008; Ellis et al., 2011; Heaton et al., 2011), the time of infection (Ettenhofer et al., 2009), the presence of other infections such as hepatitis C virus, and the use of drugs (Clifford et al., 2005; Shimizu et al., 2011).