Legionellosis is caused by bacteria called Legionella. The disease has two different forms. Legionnaires’ disease is the more severe form of infection that causes pneumonia. Pontiac fever is caused by the same bacteria, but is a milder illness without pneumonia. CDC Legionella Fact Sheet

The disease got its name after a group of people attending a convention of the American Legion in Philadelphia in 1976 developed pneumonia. While the Legionella bacteria were around before 1976, scientists had never been able to find it in an ill person’s lungs before that time.

People at risk:

Cases of legionellosis are more common in individuals who are 50 years of age or older, smokers, people with chronic lung disease, and individuals with weakened immune systems.

Mode of spread:

Legionella is naturally found in water and it is usually spread through water droplets in the air. One can get infected with Legionella when one breathes in contaminated water droplets or accidentally sucks (aspirates) water into the lungs.

There have been Outbreaks from exposure to aerosols generated form shower heads, cooling towers, mist machined, and other aerosols. Legionella grow best under a temperature of 40o C to 50o C and they have been known to survive in water chlorination. Legionella can colonize the water systems of hospitals and large buildings.

Common sources of contaminated droplets:

  • Hot tubs that are not cleaned and disinfected properly
  • Cooling towers
  • Plumbing systems
  • Hot water tanks and heaters
  • Decorative fountains


CDC Annual Report

Virginia Annual Report – Legionellosis (2016)


Water systems in buildings should be well maintained in order to reduce risk of legionella colonization and spreading. Developing a water management plan is the key to preventing legionella infections.

Want to learn more?

Contact your local Health Department or Department of Environmental Epidemiology for more information about Legionella.

Link to resource websites: