Opportunistic infection

Opportunistic infection
Chest X-ray of a patient who first had influenza and then developed Haemophilus influenzae pneumonia, presumably opportunistic
Specialty	Infectious diseases

An opportunistic infection is a serious infection caused by pathogens (bacteria, fungi, parasites or viruses) that under normal conditions, such as in humans with uncompromised immune systems, would cause a mild infection or no infection at all. 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.^[2][3][4][5] Opportunistic infections can also be attributed to pathogens which cause mild illness in healthy individuals but lead to more serious illness when given the opportunity to take advantage of an immunocompromised host.^[6]