HIV attacks the body's immune system, its defense against infections. If your immune system is damaged, you lose this defense and can develop many serious, often deadly infections and even cancers. These diseases are called 'opportunistic infections' (OIs) because they take advantage of your body's weakened defenses. e. g. Tuberculosis, Cytomegalovirus (CMV), Pnemocysis jerovecii.
Attribution: Avallain