It can take up to 10 days for symptoms to appear, and when they do, they initially look like a bad case of flu. The illness typically begins with a high fever, a cough, shortness of breath, muscle ache and headache. After a couple of days, these symptoms progress to pneumonia, a buildup of fluid in the lungs that makes it difficult to breathe. In 2014 and 2015, more than 95 percent of people with Legionnaires' disease wound up being hospitalized. While the disease is treatable with antibiotics, about 1 in 8 still died from their infection.