A variety of diseases can cause an increase in CD8 lymphocytes, such as common malignancies, diabetes, hepatitis B, tuberculosis, cytomegalovirus infections, infectious mononucleosis, and sometimes colds as well. Most commonly, patients with AIDS can also develop CD8 lymphocytosis. It is difficult to identify a particular disease based on increased CD8+ T lymphocytes alone, and even in AIDS patients with increased CD8 lymphocytes, a comprehensive analysis should be performed in combination with CD4 lymphocytes, because the AIDS virus mainly attacks CD4+ T lymphocytes. CD8 cells, after activation and differentiation, can kill cells infected by viruses and bacteria and cancer cells.