[The essence of infection by opportunistic microorganisms]

Infection is regarded as an imbalance between microbial pathogenicity factors and the host defense systems. Opportunistic infections are defined as infections rarely observed in humans with normal immune responses. The term immunocompromised (compromised) host refers to host in which one or more defence mechanisms are inactive and in which the probability of infection is therefore increased. Hospital patients are often compromised host. Compromised hosts exist even outside the hospital (smoking, intravenous drug use, poor nutrition and other). A good example is HIV (human immunodeficiency virus). HIV causes acquired immunodeficiency syndrome (AIDS) by destroying the CD4 T lymphocytes, involved in the immune response. The most common AIDS-associated opportunistic infections include pneumonia caused by the fungus Pneumocystis carinii, systemic candidiasis (Candida albicans) and cryptococcosis (Cryptococcus neoformans), protozoal infections such as cryptosporidiosis (Cryptosporidium spp.) and toxoplasmosis (Toxoplasma gondii), viral infections due to HSV, CMV, EBV, HPV or HHV8, tuberculosis and other bacterial infections. Pneumocystis carinii pneumonia (PCP) is the most common opportunistic disease observed in AIDS patients. Disease does not necessarily follow exposure to a given causal agent (pathogen 01 opportunistic pathogen). In fact, the occurrence (or otherwise) of disease typically depends on various factors--including the degree of sensitivity of the host (as above) and the virulence factors of the pathogens. Overtly aggressive products such as toxins and aggressins are clearly virulence factors. However, so too are those products and strategies which help a pathogen to become established in the host and to evade the host's defences. Certain virulence factors can be induced in the pathogen via signal transduction pathways from environment.