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We review the case definition, occurrence, and outcome of sepsis. We discuss whether the epidemiology of sepsis has changed over time and discuss issues important to our understanding of sepsis.Literature review.Our understanding of the epidemiology of sepsis is hampered by the lack of a reliable case definition. Inconsistent application of sepsis definition criteria contributes to confusion and variability in the literature. Variability in the time course of sepsis also introduces difficulty. The Centers for Disease Control estimated an incidence of 73.6 per 100,000 population in 1979, rising to 175.9 per 100,000 in 1989. However, this study was of septicemia, not severe sepsis. There are several hospital-based studies of the occurrence of severe sepsis, defined using the American College of Chest Physicians/Society of Critical Care Medicine consensus criteria. These studies reported variable hospital and intensive care unit (ICU) occurrence rates, ranging from 2% to 11% of all hospital or ICU admissions. Most of these data are from academic, tertiary care centers, which limits generalizability. More population-based studies are required to better delineate the incidence and risk factors of sepsis in the general population.Hospital mortality from sepsis has ranged from 25% to 80% over the last few decades. Although mortality may be lower in recent years, sepsis is clearly still a very serious condition. Achieving a better understanding of whether the mortality rate for sepsis is falling, however, is confounded by the lack of a uniform definition. Risk factors for adverse outcome include the degree of physiologic derangement, organ dysfunction, underlying illness, site of infection, and microbiological etiology. We do not know, however, the factors that predict response to new therapies. This dilemma has led researchers to explore whether markers of the inflammatory cascade might be more specific for sepsis, more accurate for risk prediction, or more useful for predicting response to therapy. However, there as yet is no equivalent of the CPK-MB for acute myocardial infarction. Whether we will find such a marker as we develop a greater understanding of the genetic control of the inflammatory cascade is uncertain but promising.One might assume intuitively that the epidemiology of sepsis is changing. For example, the number of patients being treated in ICUs has increased over time, the technologies used in the ICU have changed, and the choice and the use of antibiotics have changed. Predisposing factors, such as chemotherapeutic regimens, have also changed, and there have been marked changes in antibiotic resistance. Furthermore, there have been wide changes in the microbiological etiologies of diseases such as pneumonia and acute exacerbations of chronic bronchitis. However, lacking good case definitions and true incidence studies, we can only make inferences about whether the epidemiology of sepsis is truly changing.Many studies have documented many aspects of the epidemiology of sepsis. However, the composite picture they provide, although rich in many aspects, remains incomplete and emphasizes the heterogeneity of the condition. Unfortunately, few population-based prospective cohort studies exist that allow us to accurately delineate the risk factors for sepsis, its course, and its outcome. To place new information, such as the role of genetic predisposition, in the correct context, it is essential that such studies be conducted.