<i>Editorial Commentary:</i>Our Evolving Understanding of Legionellosis Epidemiology: Learning to Count

disease is also farther on the road to being cured when it breaks forth from concealment and manifests its power (Seneca, ca. 4 bc-65 ad) [1]. Legionella pneumophila burst onto the medical scene ~3 decades ago as the causative pathogen in a respiratory disease outbreak among American Legion conventioneers at Philadelphia's grand Bellevue Stratford Hotel [2]. As a result of the outbreak, 182 people became sick, 29 died, the hotel went out of business, and, ultimately, clinicians were introduced to a novel, virulent pathogen. The initial identification of legionellosis in the context of a large and virulent outbreak and the subsequent recognition of case clusters associated with cooling towers, fountains, and grocery store mist machines [3] did much to cement the perception of L pneumophila as an agent of respiratory outbreaks associated with inhalation of contaminated droplets. In the nosocomial field, such outbreaks have been associated with case-fatality rates exceeding 40% [4]. However, many clinicians now recognize that legionellosis is an important cause of sporadic community-acquired pneumonia and a relatively common cause of community-acquired pneumonia requiring admission to the intensive care unit [5]. As sensitive and noninvasive diagnostic options have expanded, so has the perceived spectrum of illness associated with Legionella species. A recent article by von Baum and colleagues [6] suggested that, when individuals with ambulatory communityacquired pneumonia are tested for legionellosis by urine antigen testing, nearly 4% may receive a diagnosis of legionellosis. Understanding of the links between the physical environment and legionellosis risk has undergone a parallel evolution. Legionella species are present in surface waters and ground water [7], and pathogenic species of Legionella frequently can be isolated from home and hospital waterdistribution systems, even in the presence of chlorine (and in the absence of identified outbreaks). The finding that monochloramination of potable water supplies (which is far more effective at killing Legionella species than is chlorination) is associated with a reduction in nosocomial legionellosis outbreaks [8] suggests that exposure to this uncommon pathogen via drinking water may actually be common. The association among advanced age, deficient immune status, and severe legionellosis likely results from a high frequency of microaspiration and the inability to clear aspirated organisms in individuals with these characteristics. In this issue of Clinical Infectious Diseases, Neil and Berkelman [9] provide a succinct and thought-provoking summary of recent trends in legionellosis in the United States. They report that the incidence of legionellosis increased markedly in the United States from 2002 through 2005. Should we be concerned that the incidence of a virulent infectious disease

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