Also Known As: Legionellosis, legionella, legionnaires disease
Legionellosis is a potentially fatal infectious disease caused by gram negative, aerobic bacteria belonging to the genus Legionella. Over 90% of legionellosis cases are caused by Legionella pneumophila, a ubiquitous aquatic organism that thrives in temperatures between 25 and 45 Â°C (77 and 113 Â°F), with an optimum temperature of 35 Â°C (95 Â°F).
Legionellosis takes two distinct forms:
- Legionnaires' disease, also known as "legion fever", is the more severe form of the infection and produces high fever and pneumonia.
- Pontiac fever is caused by the same bacteria but produces a milder respiratory illness without pneumonia that resembles acute influenza. Pontiac fever also has a spontaneous resolution.
Legionnaires' disease acquired its name in July 1976 when an outbreak of pneumonia occurred among people attending a convention of the American Legion at the Bellevue-Stratford Hotel in Philadelphia. On January 18, 1977 the causative agent was identified as a previously unknown strain of bacteria, subsequently named Legionella. Some people can be infected with the Legionella bacteria and have only mild symptoms or no illness at all.
Outbreaks of Legionnaires' disease receive significant media attention. However, this disease usually occurs as single, isolated cases not associated with any recognized outbreak. When outbreaks do occur, they are usually in the summer and early autumn, though cases may occur at any time of year. The fatality rate of Legionnaires' disease has ranged from 5% to 30% during various outbreaks and approaches 50% for nosocomial infections, especially when treatment with antibiotics is delayed. Most infections occur in those who are middle-age or older.
Patients with Legionnaires' disease usually have fever, chills, and a cough, which may be dry or may produce sputum. Some patients also have muscle aches, headache, tiredness, loss of appetite, loss of coordination (ataxia), and occasionally diarrhea and vomiting. Confusion and impaired cognition may also occur , as can a so-called 'relative bradycardia', i.e. low or low normal heart rate despite the presence of a fever.  Laboratory tests may show that patients’ renal functions, liver functions and electrolytes are deranged, including hyponatremia. Chest X-rays often show pneumonia with bi-basal consolidation. It is difficult to distinguish Legionnaires' disease from other types of pneumonia by symptoms or radiologic findings alone; other tests are required for diagnosis.
Persons with Pontiac fever experience fever and muscle aches without pneumonia. They generally recover in 2 to 5 days without treatment.
The time between the patient's exposure to the bacterium and the onset of illness for Legionnaires' disease is 2 to 10 days; for Pontiac fever, it is shorter, generally a few hours to 2 days.
L. pneumophila is specifically considered as a pathogen of the respiratory tract. Surface organelles are made up from at least forty various protein components reconcile motile bacteria.[clarification needed] Other infections have also been reported, including haemodialysis fistulae, pericarditis, and wound and skin infections. Bacteraemia is often associated with Legionnaires' disease. Intestinal infections may only occur as part of respiratory infections, and where gastrointestinal symptoms have on occasion been described.
One interesting feature in transmissive Legionella pneumophila is how monopolar flagellum is made up of the flagellin subunit FlaA. The function of the flagellum involves the development of Legionella pneumophila for "human macrophage-like cell lines and cytotoxicity to macrophages". Recent studies have shown flagellin noticed by mouse macrophages brings about cell death by the activation of cytosolic Naip5 (Birc1e) receptor.
According to the Centers for Disease Control and Prevention laboratory, antisera produced in rabbits have been prepared against all species and serogroups of Legionella and have been used in the CDC laboratory to distinguish the most Legionella strains in slide agglutination tests. Although, the development of an antigen that can diagnose infections with all species and serogroups of Legionella has not yet been discovered. 
Infections of protozoa such as Hartmannella vermiformis and related protozoa have been shown to be able to support the growth of L. pneumophila in tap water. Also Acanthamoeba, Naegleria and Tetrahymena can be infected by L. pneumophila. This pathway may be how these organisms survive in the environment.
Infection normally occurs after inhaling an aerosol (fine airborne particles) containing Legionella bacteria. Such particles could originate from any infected water source. When mechanical action breaks the surface of the water, small water droplets are formed, which evaporate very quickly. If these droplets contain bacteria, the bacteria cells remain suspended in the air, invisible to the naked eye and small enough to be inhaled into the lungs. This often occurs in poorly ventilated areas such as prisons where a condensating air conditioner can spread it throughout the entire room, infecting anyone not immune to the strain of bacteria.
Potential sources of such contaminated water include cooling towers (some 40% to 60% of ones tested) used in industrial cooling water systems as well as in large central air conditioning systems, evaporative coolers, nebulizers, humidifiers, whirlpool spas, hot water systems, showers, windshield washers, architectural fountains, room-air humidifiers, ice making machines, misting equipment, and similar disseminators that draw upon a public water supply.
The disease may also be transmitted from contaminated aerosols generated in hot tubs if the disinfection and maintenance program is not done rigorously. Freshwater ponds, creeks, and ornamental fountains are potential sources of Legionella. The disease is particularly associated with hotels, fountains, cruise ships and hospitals with complex potable water systems and cooling systems.
The development of bacterial infections may cause Legionnaires’ disease. Respiratory care devices such as humidifiers and nebulizers used with contaminated tap water may contain Legionella. Using sterile water is very important, especially when using respiratory care devices.