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The development of any kind of infection involves a close interplay between three elements: (1) a susceptible host, (2) an infectious agent, and (3) a suitable medium or environment. The degree of contribution of each of the three factors to the eventual occurrence of the infection depends on the individual patient and the specific nature and site of the infection. Whether a given inoculum—bacterial, viral, or fungal in nature—results in an established infection or not depends on the virulence of the bacteria, the strength of the immune and inflammatory host response (ie, chemotaxis, phagocytosis, B- and T-lymphocyte activation), and the amount of blood perfusion and oxygen tension in the medium where the inoculum resides.
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INFLAMMATION, INFECTION, & SEPSIS
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The body’s immunologic defense is composed of two components, the innate and the adaptive systems. The innate, or nonspecific, defense consists of physical barriers, phagocytic cells, and proteins. The adaptive, or specific, defense develops in response to prior exposure or from receipt of preformed antibody. The innate system is triggered in response to any potential perceived insult to the body, whether through injury, exposure, or infection. A critical part of the innate defense is the activation of a protective inflammatory response. The characteristics of inflammation include redness, swelling, warmth, pain, and loss of tissue function. These are a result of increased blood flow in conjunction with increased vascular permeability, which allows recruitment of response cells to repair and regenerate affected tissue as part of a localized response.
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Infection occurs when a microbe acquires a host. However, the severity of the clinical manifestation of that infection depends on a variety of factors including microbial factors, host factors (including the aforementioned defense systems), and the host-microbe interface. It is important to distinguish infection from inflammation, as these terms are often colloquially interchangeable. Inflammation is the host’s localized response to the infection caused by the inoculum.
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Recognizing sepsis is important because it is the primary cause of mortality from infection. In recent years, the clinical criteria for sepsis have been evolving. Efforts to better define sepsis have largely been spearheaded by the European Society of Intensive Care Medicine (ESICM) and the Society of Critical Care Medicine (SCCM), with the most recent update, known as Sepsis-3, having been published in 2016. As of this time, the Third International Consensus Definitions Task Force, composed of a team of intensivists, infectious disease specialists, surgeons, and pulmonologists, defines sepsis as a “life-threatening organ dysfunction due to a dysregulated host response to infection.” Under Sepsis-3, clinical criteria for sepsis include the following: (1) suspected or confirmed infection and (2) acute organ dysfunction, defined as an increase of two or more points from baseline on the Sequential Organ Failure Assessment (SOFA) ...