(a) : $AIDS$ (acquired immuno deficiency syndrome) a syndrome, is caused by the retrovirus $HIV$ (human immunodeficiency virus). The virus destroys a subgroup of lymphocytes, the helper $T$-­cells (or $CD4$ lymphocytes), resulting in suppression of the body’s immune response. $HIV$ is transmitted in blood, semen and vaginal fluid; the major routes of infection are unprotected vaginal and anal intercourse, intravenous drug abuse, and the administration of contaminated blood and blood products. A combination of antiviral drugs can delay the development of full­blown $AIDS$ for many years but cannot fully care the disease.