An infection caused by pathogens (bacteria, viruses, fungi, or protozoa) that take advantage of an opportunity not normally available, such as a host with a weakened immune system (e.g. AIDS, leukemia), an altered microbiota (such as a disrupted gut microbiota, e.g. Clostridium difficile infection), or breached integumentary barriers (skin damage, e.g. penetrating trauma). Many of these pathogens do not cause disease in a healthy host that has a normal immune system. Cf. dysbiosis