These opportunities can stem from a variety of sources, such as a weakened immune system (as can occur in acquired immunodeficiency syndrome), when being treated with immunosuppressive drugs (as in cancer treatment),[1] when a microbiome is altered (such as a disruption in gut microbiota), or when integumentary barriers are breached (as in penetrating trauma).

Many of these pathogens, such as the bacterium Clostridioides difficile, can be present in hosts with uncompromised immune systems without generating any symptoms, and can, in some cases, act as commensals until the balance of the immune system is disrupted.

[40][41][42][43] HIV is a virus that targets T cells of the immune system and, as a result, HIV infection can lead to progressively worsening immunodeficiency, a condition ideal for the development of opportunistic infection.

[46][47] Kaposi's sarcoma, a virally-associated cancer, has higher incidence rates in HIV-positive patients than in the general population.

[citation needed] Opportunistic infections caused by feline leukemia virus and feline immunodeficiency virus retroviral infections can be treated with lymphocyte T-cell immunomodulator.