Legionnaire’s disease is prevalent worldwide. This is respon-sible for 2–15% of all community-acquired pneumonia that requires hospitalization.
L. pneumophila is an obligate intracellular pathogen found inalveolar macrophages in the infected human host.
L. pneumophila is a saprophyte widely found in aquaticenvironments. The bacteria have been isolated from a variety of man-made water environments, such as hot water system in hotels and hospitals, cooling water system, air conditioning cooling tow-ers, shower heads, industrial coolants, and respiratory ventilators.
The factors that contribute to the presence of these bacteria in these environments include (a) stagnation of water, (b) tem-perature of water varying from 25 to 40°C, and (c) presence of free-living amoeba, such as Acanthamoeba andBartonella species, which support the growth of Legionella. Therefore, contami-nated water is the primary source of infection. Transmission to humans occurs by the following routes:
· Inhalation of aerosolized mist from contaminated water source, which is contaminated with either the bacteria or free-living amoeba infected with the bacteria.
· Nosocomial infection is transmitted through aspiration, contaminated water, or respiratory equipments.
· Infection is also transmitted by the use of nebulizers, humidifiers, and other instruments that have been washed with contaminated tap water.
Person-to-person transmission does not occur. Animals also do not play any role in transmission of the infection. The elderly people, people with chronic heart or lung diseases, and the patients who are in immunocompromised state or are receiving immunosuppressive therapy are at greatest risk for the disease. This is because of their depressed cell-mediated immunity and altered pulmonary functions.