Legionella is a genus of gram-negative bacteria that can be seen using a silver stain or grown in a special media that contains cysteine, an amino acid.

Legionella was named after a 1976 outbreak of a then-unknown "mystery disease" at a convention of the American Legion, an association of U.S. military veterans, in Philadelphia.

This outbreak happened within days of the 200th anniversary of the signing of the Declaration of Independence, which led to it being highly publicized and caused great concern in the U.S.[6] On January 18, 1977, the causative agent was identified as a previously unknown bacterium subsequently named Legionella.

To detect Legionella in water, it is first concentrated, then inoculated into charcoal yeast extract agar containing selective agents that prevent the growth of other organisms.

Methods, like polymerase chain reaction (PCR) and rapid immunological tests, can detect Legionella in water much faster.

[10] Government health surveillance reports have shown an increase in the proportion of water-related Legionella outbreaks, particularly in healthcare settings.

[23] Cooling towers are well established as sources of Legionella that may have an effect on community exposure to the bacterium and Legionnaires' disease epidemics.

[28] Weather patterns and other environmental factors may increase risk of Legionella outbreaks; a study in Minnesota, USA, using outbreak information from 2011 to 2018 showed precipitation as having the greatest effect of increasing risk of Legionella exposure when taking into account other environmental factors (temperature, relative humidity, land use and age of infected person).

Protection was dose-dependent and correlated with antibody levels as measured by enzyme-linked immunosorbent assay (ELISA) to an outer membrane antigen and by indirect immunofluorescence to heat-killed cells.

The early symptoms are general, including fever, muscle pain, headache, shortness of breath, and a dry or productive cough.

A physical examination may reveal abnormal lungs sounds such as rales or rhonchi, and if consolidation is present, there may be signs like egophony or dullness to percussion.

[33] In nature, Legionella bacteria live inside tiny organisms, like amoebae (examples: Acanthamoeba spp., Naegleria spp., Vermamoeba spp., or other protozoa such as Tetrahymena pyriformis[34]).

People with chronic illnesses, like autoimmune disease treated with TNF inhibitors, also face a higher risk of infection.

After inhaling or accidentally swallowing small aerosol particles, Legionella bacteria attach to immune cells and are taken up by them through a process called phagocytosis.

[33] Legionella has several ways to evade the immune system, increasing the chance that a person develops symptoms of infection.

It also reduces the activity of cytokine receptors (which play a role in immune response), blocks the production of certain proteins needed by the host, and avoids being broken down by lysosomes, which are cell structures meant to digest harmful particles.

Some patients, especially those in the ICU or those who cannot provide a sputum sample, may need an invasive procedure, such as a bronchoscopy, if the initial urinary antigen test is negative.

[33] Preventing Legionella infection starts with improving water systems and setting up water-monitoring processes to keep it under control.

Facilities with water features, like therapy pools, ice machines, and decorative fountains, must have cleaning and disinfection policies.

Using disinfectants requires regular maintenance and monitoring of chemical levels to ensure they're effective in preventing Legionella growth.

For patients who are immunosuppressed or have severe cases of Legionella pneumonia, a longer treatment course of three weeks is recommended to ensure effective recovery.

The case fatality rate for this type of pneumonia is about 10%, and patients who are admitted to the ICU or have other major health issues are more likely to die from it.

[33] Examples of common-source outbreaks:[33] Control of Legionella growth can occur through chemical, thermal or ultraviolet treatment methods.

For systems with significant Legionella problems, temporary shock chlorination—where levels are raised to higher than 2 ppm for a period of 24 hours or more and then returned to 0.5 ppm—may be effective.

[45] Hyperchlorination can also be used where the water system is taken out of service and the chlorine residual is raised to 50 ppm or higher at all distal points for 24 hours or more.

Copper-silver ionization is an effective process to control Legionella in potable water distribution systems found in health facilities, hotels, nursing homes, and most large buildings.

[51] It is also not widely distributed in water systems due to concerns regarding its toxicity, unpleasant odors, and harmful byproducts, as stated above, that it can create.

Failure to show monitoring records to at least this standard has resulted in several high-profile prosecutions, e.g. Nalco + Bulmers – neither could prove a sufficient scheme to be in place while investigating an outbreak, therefore both were fined about £300,000GBP.

Important case law in this area is R v Trustees of the Science Museum 3 All ER 853, (1993) 1 WLR 1171[56] Employers and those responsible for premises within the UK are required under Control of Substances Hazardous to Health to undertake an assessment of the risks arising from Legionella.

[59][60][61] A former Soviet bioengineer, Sergei Popov, stated in 2000 that his team experimented with genetically enhanced bioweapons, including Legionella.