Legionella bacteria are microscopic organisms that live in soil and water and are the most common cause of Legionnaires' disease, explains Mayo Clinic. The same organism causes Pontiac fever, which is a milder form of the disease that can usually resolve without treatment.
Legionnaires' disease can be fatal without treatment, states Mayo Clinic. The disease is treated with antibiotics, which destroy the legionella bacteria. It can take between two and 10 days before the symptoms of Legionnaires' disease begin, and initial symptoms include muscle pain, high fever, headache and chills. A day or two later, new symptoms can appear including shortness of breath, nausea, coughing, chest pains and confusion. The coughing can produce phlegm and sometimes blood, and the nausea may be accompanied by vomiting and diarrhea.
Legionella bacteria most commonly infect the lungs, but they can spread to the heart and other organs, according to Mayo Clinic. Possible complications of a legionella bacteria infection include septic shock, respiratory failure and acute kidney failure, all of which are life-threatening. Treatment often must occur in a hospital, and the sooner it begins, the less likely severe complications occur. Severe legionella bacteria infections are most likely to occur in people with weakened immune systems, smokers, people with chronic lung disease and people over 50 years old.