legionnaires disease is a serious lung infection caused by legionella bacteria initial symptoms usually include flu like symptoms such as mild headaches muscle pain high temperature fever usually 38c 100 4f or above chillstiredness changes to your mental state such as confusionntroduction legionnaires disease is a serious lung infection caused by legionella bacteria initial symptoms usually include flu like symptoms such as mild headaches muscle pain high temperature fever usually 38c 100 4f or above chills tiredness