An opportunistic infection is an infection caused by pathogens (bacteria, fungi, parasites or viruses) that take advantage of an opportunity not normally available. These opportunities can stem from a variety of sources, such as a weakened immune system (as can occur in acquired immunodeficiency syndrome or when being treated with immunosuppressive drugs, as in cancer treatment), an altered microbiome (such as a disruption in gut microbiota), or breached integumentary barriers (as in penetrating trauma). Many of these pathogens do not necessarily cause disease in a healthy host that has a non-compromised immune system, and can, in some cases, act as commensals until the balance of the immune system is disrupted. 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.
A wide variety of pathogens are involved in opportunistic infection and can cause a similarly wide range in pathologies. A partial list of opportunistic pathogens and their associated presentations includes:
Clostridioides difficile (formerly known as Clostridium difficile) is a species of bacteria that is known to cause gastrointestinal infection and is typically associated with the hospital setting.
Legionella pneumophila is a bacterium that causes Legionnaire’s disease, a respiratory infection.
Mycobacterium avium complex (MAC) is a group of two bacteria, M. avium and M. intracellulare, that typically co-infect, leading to a lung infection called mycobacterium avium-intracellulare infection.
Mycobacterium tuberculosis is a species of bacteria that causes tuberculosis, a respiratory infection.
Pseudomonas aeruginosa is a bacterium that can cause respiratory infections. It is frequently associated with cystic fibrosis and hospital-acquired infections.
Salmonella is a genus of bacteria, known to cause gastrointestinal infections.
Staphylococcus aureus is a bacterium known to cause skin infections and sepsis, among other pathologies.
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Feline immunodeficiency virus (FIV) is a Lentivirus that affects cats worldwide, with 2.5% to 4.4% of felines being infected. FIV was first isolated in 1986, by Niels C Pedersen and Janet K. Yamamoto at the UC Davis School of Veterinary Medicine in a colony of cats that had a high prevalence of opportunistic infections and degenerative conditions and was originally called Feline T-lymphotropic virus. It has since been identified in domestic cats. It has been suggested FIV originated in Africa and has since spread to feline species worldwide.
Feline leukemia virus (FeLV) is a retrovirus that infects cats. FeLV can be transmitted from infected cats when the transfer of saliva or nasal secretions is involved. If not defeated by the animal's immune system, the virus weakens the cat's immune system, which can lead to diseases which can be lethal. Because FeLV is cat-to-cat contagious, FeLV+ cats should only live with other FeLV+ cats. FeLV is categorized into four subgroups, A, B, C and T. An infected cat has a combination of FeLV-A and one or more of the other subgroups.
Bacteria (bækˈtɪəriə; : bacterium) are ubiquitous, mostly free-living organisms often consisting of one biological cell. They constitute a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria were among the first life forms to appear on Earth, and are present in most of its habitats. Bacteria inhabit soil, water, acidic hot springs, radioactive waste, and the deep biosphere of Earth's crust. Bacteria play a vital role in many stages of the nutrient cycle by recycling nutrients and the fixation of nitrogen from the atmosphere.
JC polyomavirus (JCPyV) is an opportunistic virus that remains in a latent state in the kidneys and lymphoid organs of more than half of the human adult population. In rare cases of severe immune suppression, the virus is able to establish a lytic infectio ...
The opportunistic human pathogen Pseudomonas aeruginosa is the causal agent of a wide variety of infections. This non-fermentative Gram-negative bacillus can colonize zones where the skin barrier is weakened, such as wounds or burns. It also causes infecti ...
The Mycobacterium tuberculosis complex (MTBC) comprises nine human-adapted lineages that differ in their geographical distribution. Local adaptation of specific MTBC genotypes to the respective human host population has been invoked in this context. We aim ...