Which bacterium causes Legionnaires’ disease, a severe pneumonia often linked to contaminated water systems?

Legionella pneumophila causes Legionnaires’ disease, a severe pneumonia often linked to contaminated water systems.

Legionella pneumophila is a Gram-negative bacterium that naturally lives in freshwater environments, where it can multiply inside amoebae and other protozoa. Human infections usually occur when contaminated water is aerosolized and inhaled, such as through showers, cooling towers, whirlpool spas, or complex building plumbing. People do not typically catch Legionnaires’ disease by drinking water or through ordinary person-to-person spread.

The disease was recognized after a 1976 outbreak among attendees of an American Legion convention in Philadelphia, which gave the illness its name. Symptoms can include fever, cough, breathing difficulty, muscle aches, headache, and gastrointestinal problems.

A related, milder illness called Pontiac fever is also associated with Legionella, but it does not usually cause pneumonia. Legionella longbeachae is another human pathogen linked especially to potting mix, so it is a plausible but distinct answer.