This enhanced CD4+ T cell response may be of particular importance in the control of chronic viral infections (60), the maintenance of CD8+ T cells during prolonged viral infections (61) where CD4+ T cell help may be required for optimal CD8+ T cell activity (62) or for CD8+ T cell memory (63), or indeed for optimal responses against any pathogens where a CD4+ T cell vaccine induced response is required. Here, CD4 is linked to viral infectious disease.