Legionnaires' disease, also called Legionellosis is a lung infection, a form of pneumonia which can be fatal, caused by the legionella bacterium. The name Legionnaires' disease emerged in 1976 - in July there was an outbreak of pneumonia among delegates at a convention of the American Legion in Philadelphia.s Legionnaires' disease signs and symptoms are often mild, it is likely that the real number of infections is considerably higher than the reported ones. Legionnaires' disease may be hard to diagnose at first because its signs and symptoms may be very similar to other forms of pneumonia.