HIV is spherical virus with a diameter of about 90-120 nm. Its genome consists of a single-stranded RNA filament segmented into two identical filaments and associated with reverse transcriptase enzymes. The envelope consists of a lipid bilayer derived from host cell membrane and projecting knob like glycoprotein spikes. It contains two protein coats. HIV is a retrovirus that attacks helper T cells. Without an adequate supply of helper cells, the immune system cannot signal B cells to produce antibodies to kill infected cells, thus body becomes susceptible to infectious. This immune deficiency is described the name acquired immune deficiency syndrome or AIDS.