The AIDS is a fatal disease caused by the  retrovirus known as Hummun immuno deficiency virus(HIV).This virus breaks the immunity system of the body.AIDS refers only to the last stage of HIV infection. It is also called a modern pandemic effecting both industrialized and developing countries.The AIDS was first recognized in USA in 1981.HIV was earliness known as human T-cell lymphotropic virus 3.It has a protein capsule.The genome of this virus is diploid.it is composed of 2 identical RNA copies.Reverse transcriptase enzyme is the characteristic feature of retro virus.The he virus remains in the body once it is infected.The  modes of transmission of this virus are  transfusion of infected blood, use of contaminated needles and syringes, and also by the use of contaminated razors.This virus can also be transmitted by sexual  intercourse with the infected identity, artificial insemination and by organ transplantation.This cannot be acquired by crowd transport and shaking hands and kissing etc..The AIDS virus consist of glycoprotien ceat ,double layer of lipid membrane and two protein protein bands.It also contain RNA and reverse transcriptise enzyme.  The human chromosome get inserted by the DNa copy of HIV and this replicates the DNA cell..This results in the infection of the virus particles in the new  T-helper cells. .The major symptoms of aids includes fever , lethargy , pharyngitis, naucea and headache..The AIDS can be diagnosed by ELIZA TEST.The incubation period of AIDS is between six months to ten years .