Legionnaires’ disease is a respiratory disease that occurs when the bacteria Legionella pneumophila infect the lungs. In order to become sick, you have to inhale microscopic droplets of water that are contaminated with the bacteria. Simply drinking contaminated water is not enough to make you sick, and you cannot catch the disease from someone else who is sick.

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% 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.

Although the bacteria can infect people of all ages, more than 80% of reported cases were 50 years old or older and about 60% were men. Smokers and people with underlying lung diseases – such as emphysema or chronic obstructive pulmonary disease – or weakened immune systems, due to medicines or health problems such as cancer or diabetes, are also at higher risk of infection.