Illustration of the retrovirus HIV-1 (centre) responsible for AIDS, Acquired Immune Deficiency Syndrome, and a target T-helper lymphocyte cell. HIV-1 has at its centre strands of RNA surrounded by a pink protein membrane. The outer envelope is crossed by yellow spikes of glycoprotein gp120 which will link the virus to the CD4 cells (pink) on the surface of the lymphocyte. The lymphocyte becomes infected and may die or the virus may remain dormant with the possibility of later reactivation. The progressive destruction of T-helper cells leads to a severe deficiency in the immune system which makes people affected more prone to opportunistic infections and cancers.