HIV attacks the immune system destroying helper T cells to suppress cell-mediated immunity. Upon suppression, immune system loses the capability to fight against opportunistic infections. AIDS is the last stage of HIV infection characterized by <200 CD4 cells/microliter, lymphadenopathy, oral candidasis, hairy leukoplaskia, recurrent bacterial infections, pneumocystitis pneumonia and tumors e.g. Kaposi's sarcoma.