(a) : Common cold is a viral disease. It is caused by Rhino viruses. It causes fever and pain all over the body and affects the nose, throat and air passages. $AIDS$ (Acquired immuno deficiency syndrome) is a disorder of cell mediated immune system of the body. It is caused by $HIV$ (Human immunodeficency virus). $HIV$ is a retrovirus that attacks helper $T$-­cells.