Natural killer (NK) cells are derived from pluripotent (capable of giving rise to several different cell types) hematopoietic stem cell precursors which develop independently of the thymus. NK cells play a vital role in host defense against viral infections and tumor surveillance as well. NK cells are important constituent of the innate immune response against viral infections. NK cells also influence the course of chronic viral infections, such as infections with HIV-1 and hepatitis C virus (HCV). These infections at a chronic stage have a negative impact on NK cell function and leads to the appearance of phenotypically and functionally abnormal NK cells.