Legionella is a genus "Genus (biology)") of gram-negative, bacillus-shaped bacteria.[1]​ They live in stagnant waters with a wide temperature range, preferably above 35 °C. Its growth is favored by the presence of organic matter. They have aerobic respiration and a flagellum to move. Within this genus there are 48 species and a total of about 78 serotypes. Some of the legionella species can infect humans. The most important species in this aspect is Legionella pneumophila due to its medical implications. Legionella can multiply within amoebas and its coculture is sometimes the best known method to detect its infectious presence.[2]​.

Legionella infection is called legionellosis, which can present as a febrile illness, either mild and without pulmonary focus called Pontiac fever, or severe as an atypical pneumonia called Legionnaires' disease.[3] It can lead to pulmonary complications and its symptoms are: fatigue, difficulty breathing and sometimes diarrhea or muscle pain. Many exposed to the germ do not get sick.[4]​ The greatest source of contagion is the water system of large buildings, hotels and hospitals, humidifiers, mist machines, spas and thermal water sources. Air conditioning systems are also a relevant source of legionella.

For its identification, microbiological analysis is required through:.

Although different autonomous communities have specific legislation for them, the prevention and control of the disease at the national level is legislated by two main laws:

In Spain, Legionella control companies must have a certificate of registration with the Ministry of the Environment and additional certificates such as: