Most patients develop pneumonia, a condition in which some of the lungs' air sacs fill with fluid or pus, so air is excluded. The pneumonia might involve both lungs and become so severe that hospitalization is required. Pneumonia resulting from Legionnaires' disease is similar to pneumonia from other causes. Laboratory tests are necessary for a confident diagnosis. Mental changes, such as confusion, disorientation, hallucination and loss of memory, can occur to an extent that seems out of proportion to the seriousness of fever.
Complete recovery can take several weeks.
Legionnaires' disease may be suspected from the clinical features that seem typical of Legionella pneumonia. These features include diarrhea and symptoms attributable to the central nervous system, particularly confusion and loss of memory. To distinguish Legionnaires' disease from pneumonia from other causes, special laboratory tests are needed that, normally, are not carried out on patients with fever and pneumonia. An increase indicates infection by Legionella. Antibodies are produced by special cells of the body's infection defense system to reduce or neutralize the effect of invading microorganisms.