What is Legionnaires’ disease? Legionnaires’ Disease is a type of pneumonia with atypical (other than typical) symptoms in the lungs. It is a dangerous disease with a “High Mortality Rate”, which has a fatal effect especially in advanced age groups, as it progresses rapidly and seriously damages the lung capacity. It takes its name from the professional characteristics of the people who have the disease. To shed some light on this issue, it would be correct to first talk about the meaning of the word Legionnaire.