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The aims of diagnosis in infectious diseases are (1) to recognize that a sick patient is ill with an infectious disease and (2) to identify the specific etiologic agent in order to determine the most appropriate treatment, as the susceptibility of most pathogens to selected antimicrobial agents is known.

Identification of the infectious agent may be achieved in several ways.

Clinical History

Prevalence of Infectious Diseases

Hospital-Acquired (Nosocomial) Infections

Of the approximately 5 million cases of infectious diseases seen in hospitals annually in the United States, about 40% are acquired in hospitals (nosocomial infections). Hospital-acquired infections are different from those acquired in the community outside the hospital (Table 14-1). The genitourinary tract, surgical wounds, and lungs are the most common sites of nosocomial infection. Most nosocomial infections are caused by bacteria and fungi; viruses, protozoa, and metazoa rarely cause such infections. Hospital-acquired pneumonia is the most serious such infection, with a mortality rate of 20%. Disseminated infections caused by bacteria and fungi, mainly in immunocompromised patients, also account for a significant number of nosocomial infections. In newborn nurseries, hospital-acquired epidemic diarrhea is a common problem.

Table 14–1. Common Infections Seen in Hospitals in the United States. 1

The high incidence of hospital-acquired infections is due to the following factors:

Increased Susceptibility

Hospitalized patients demonstrate increased susceptibility to infection as a consequence of debilitation associated with concurrent illness and in some cases the administration of immunosuppressive drugs.

Use of Invasive Procedures

Invasive surgical, diagnostic (eg, lumbar puncture, phlebotomy), and therapeutic procedures (eg, bladder catheterization, indwelling intravascular catheters) provide ...

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