People of any age can develop Legionnaires' disease, but the illness most often affects middle-aged and older people, particularly those who smoke cigarettes or have chronic lung disease, as these individuals have a greater likelihood of developing any respiratory illness.People at an increased risk for Legionnaires' disease also include people whose immune systems are suppressed by diseases and conditions such as cancer, kidney failure requiring dialysis, diabetes, or AIDS. Those who take medications that suppress the immune system are also at risk including those chronically on steroids.Pontiac fever most commonly occurs in healthy people.