Legionellosis (Legionnaires’ Disease and Pontiac Fever)

Report within 24 hours of Diagnosis to the Local Health Department.

Legionnaires' disease and Pontiac fever are collectively known as legionellosis, a disease caused by Legionella bacteria. Legionella is a type of bacterium found naturally in freshwater environments, like lakes and streams. It can become a health concern when it grows and spreads in human-made water systems such as building premise plumbing and cooling towers (structures that contain water and a fan as part of centralized air-cooling systems for building or industrial processes). Legionella can continue to persist in the water system unless proper steps are taken to prevent the growth of bacteria.

People can get Legionnaires’ disease or Pontiac fever when they breathe in small droplets of water in the air that contain the bacteria. People can breathe in small droplets of water by using a shower, hot tub, or sink. Other sources of aerosolized water include decorative fountains and cooling towers.

Less commonly, people can get sick by aspiration of drinking water containing Legionella. This happens when water accidently goes into the lungs while drinking. People at increased risk of aspiration include those with swallowing difficulties.

Note: Home and car air-conditioning units do not use water to cool the air, so they are not a risk for Legionella growth.


Education Materials
Trainings & Webinars
  • Legionella Investigation for Local Health Departments webinar slides (6/22/21)
  • A Deeper Dive into Legionella Outbreak Investigation for Local Health Departments webinar slides (12/01/21) 
Last Reviewed: 7/27/2023