infections, and patients who recover from these infections typically require 10 extra days of hospital care (Fuchs, 1979; Wenzel, 1988). Every year, hospital-acquired infections account for between $5 billion and $10 billion in additional medical-related expenses, most of which are due to excess hospital stays (Wenzel, 1987; Schaechter et al., 1989; Martone, 1990).

Although hospital sanitation has improved markedly since the late nineteenth century, when carbolic acid was first used as an antiseptic during surgery, nosocomial infections continue to challenge efforts to control them (Fuchs, 1979). Medical advances and antimicrobial resistance are at the heart of the struggle.

In February 1991, the CDC's Hospital Infections Division looked at 10-year trends in nosocomial infections using data collected through the National Nosocomial Infections Surveillance System (NNISS). The 1980s saw a tripling of the incidence of bacteremias (Ross, 1990) and a shift in the organisms that are most prevalent as the causes of nosocomial infections, from those that are generally susceptible to antimicrobials (e.g., Proteus mirabilis, Escherichia coli, and Klebsiella pneumoniae) to those that tend to be more refractory to treatment (e.g., Enterobacter, Pseudomonas, Enterococcus, and Candida species) (Schaberg et al., 1991). In addition, there appears to have been a significant increase in both the prevalence and variety of viral and fungal pathogens found to be causes of nosocomial infections (Ross, 1990). All of these observations implicate the hospital setting as a prime site for the emergence of microbial threats to health.

Many of the factors that increase the risk of infection in a hospital are inherent to any health care setting. Not only are persons with serious infections frequently admitted to hospitals, thus providing an intrahospital source of pathogenic organisms, but the proportion of people with increased susceptibility to infections is also greater in a hospital than in the general population. In addition, because health care institutions are not completely isolated from the community (employees, visitors, food, and supplies enter daily), patients are exposed to the same pathogens that circulate in the surrounding locale. Thus, nosocomial infections can be transmitted from staff to patients, from visitors to patients, and from patients to other patients. Infections can also be acquired from contaminated surfaces, such as floors, examining tables, or improperly sterilized instruments, and from the patient's own normal microbial flora, especially during invasive procedures.

Antimicrobial resistance, a problem in the treatment of many bacterial diseases, has particular relevance in the hospital setting. By their very nature, hospitals are filled with people who have increased susceptibility to infection. Also by nature, hospitals tend to use large quantities of antibiotics. (About a third of hospitalized patients receive such agents [Shapiro et al., 1979].) The combination of an immunologically vulnerable population and the widespread use of antibiotics is potentially risky, since the selective



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