Opportunistic pathogens are microorganism that normally does not cause disease but nevertheless can cause disease under certain relatively unusual circumstances such as that provided by a weakened or compromised immune system (e.g. in HIV/AIDS infected individuals). They are microorganisms that under normal circumstances in healthy hosts do not cause disease. However, they act as commensals, but when given opportunity due to underlying injury or disease such as immunodeficiency, they can cause disease or infections called opportunistic infections. Opportunistic infections are infections caused by opportunistic pathogens in people with weakened immune system.
Commensals are organisms that are living in close association with another organism of a different species where neither has an obvious effect on the other. They can be referred to as the normal microflora of the body. Commensal microflora (normal microflora, indigenous microbiota) consists of those micro-organisms, which are present on body surfaces covered by epithelial cells and are exposed to the external environment (gastrointestinal and respiratory tract, vagina, and the skin). Opportunistic pathogens often are otherwise benign members of an individual’s normal flora, though they can also be relatively common environmental organisms not found in the human body. They only cause infection in individuals with a weakened or compromised immune system. An opportunistic pathogen basically needs to be in the right place at the right time to cause disease but normally these circumstances do not coincide and the organism consequently is otherwise harmless.
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