An atypical type of pneumonia that has been isolated from water supplies, humidifiers and air conditioning equipment is caused by

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Legionella pneumophila is the organism classically linked to water systems and aerosolized water, such as showers, cooling towers, humidifiers, and air conditioning equipment. Inhalation of these contaminated aerosols can cause Legionnaires’ disease, a form of atypical pneumonia. This makes the water- and aerosol-associated environment a hallmark clue for this pathogen. Legionella thrives in man-made water systems and is not typically spread from person to person, which helps distinguish it from other causes of pneumonia.

For context, other organisms listed fit different patterns: Mycoplasma pneumoniae also causes atypical pneumonia but is mainly spread through close respiratory contact rather than water sources; Streptococcus pneumoniae is the classic cause of typical bacterial pneumonia and isn’t tied to water systems; Chlamydia psittaci is associated with birds (psittacosis) rather than water aerosols.