Legionnaire disease is a type of pneumonia. It is one of a group of diseases (known collectively as legionellosis) that can occur after exposure to Legionella bacteria, most often Legionella pneumophila. Legionella bacteria are found naturally in the environment and are present in low levels in streams, rivers and ponds. However, these bacteria can grow to dangerous levels in the warmer water associated with hot water tanks, large air-conditioning systems, hot tubs and cooling towers. Breathing in the vapor of contaminated water can lead to Legionnaire disease or a milder form of infection called Pontiac fever.

The symptoms of Legionnaire disease often include moderate to severe pneumonia coupled with gastrointestinal problems such as diarrhea and vomiting. Because it is difficult to differentiate between Legionnaire disease and other forms of pneumonia, cases of Legionnaire disease are often underreported. Legionnaire disease can be treated with antibiotics.

Pontiac fever is a much milder infection of Legionella bacteria. It produces mild flu-like symptoms with no pneumonia. It is a self-limiting disease, which means that the bacteria stop multiplying after a certain amount of time.

Although there are few steps that individuals can take to avoid Legionella bacteria, the U.S. Centers for Disease Control and Prevention has suggested guidelines for building managers to reduce outbreaks. These include the regular maintenance, filtering and disinfecting of water storage and supply systems.