Opportunistic Infections (OIs)OIs are illnesses that take advantage of a person’s weakened immune system. OIs do not normally appear in persons with healthy immune systems. The Centers for Disease Control and Prevention (CDC) have generated a list of 40 OIs, which includes: Kaposi’s sarcoma, pneumocystis jiroveci pneumonia, Toxoplasmosis, Cryptococcal Meningitis, Mycobacterium Avium Complex (MAC), Cytomegalovirus retinitis, among others.