Legionnaires' disease, also known as legionellosis or Legion fever, is a form of atypical pneumonia caused by any type of Legionella bacteria. Over 90% of cases of Legionnaires' disease are caused by Legionella pneumophila.Other causative species include L. longbeachae, L. feeleii, L. micdadei, and L. anisa. These species cause a less severe infection known as Pontiac fever, which resembles acute influenza. These bacterial species can be water-borne or present in soil, whereas L. pneumophila has only been found in aquatic systems, where it is symbiotically present in aquatic-borne amoebae. It thrives in temperatures between 25 and 45 °C (77 and 113 °F), with an optimum temperature of 35 °C (95 °F). During infection, the bacterium invades macrophages and lung epithelial cells and reproduces within the infected cells.