Legionellosis (Legionnaires' Disease and Pontiac Fever)

Legionellosis in an infection caused by Legionella bacteria.

There are two different types of legionellosis:

  • Pontiac fever
  • Legionnaires' disease

Pontiac fever is a mild respiratory illness and Legionnaires' disease is more severe and is a type of pneumonia.

Legionnaires' disease is not normally spread from person to person.

Side view of a running shower head


The Department of Natural Resource’s Drinking Water and Groundwater Program and the Department of Health Services are advising all facilities, including schools, to consider the safety of their water supply. Many schools, gyms, libraries, and other educational facilities across the state have been temporarily shut down or experienced reduced usage in response to the COVID-19 pandemic. Before opening back up to the public or re-starting general use, it’s important to flush any stagnant water out of the plumbing system.

When water systems are dormant, there is a greater chance that bacteria can grow throughout the plumbing system. Of particular concern is the bacteria Legionella, which can cause a deadly respiratory infection with severe pneumonia-like symptoms, especially in people over 50 years old or those with compromised immune systems. When water is stagnant, hot water temperatures can decrease to the Legionella growth range (77–108°F, 25–42°C). Stagnant water can also lead to low or undetectable levels of disinfectant, such as chlorine. Ensure that your water system is safe to use after a prolonged shutdown to minimize the risk of Legionnaires’ disease and other diseases associated with water. In August, several schools in Ohio and Pennsylvania reported the detection of Legionella after sampling water systems that had been stagnant.

The Centers for Disease Control and Prevention recently published Guidance for Reopening Buildings After Prolonged Shutdown or Reduced Operation. Included in the guidance are 8 steps to follow to minimize Legionella risk before reopening a building.

Flushing stagnant water is just one of the several helpful recommendations. Additional steps building managers or owners can take include checking water heaters to make sure they are maintained, and inspecting and cleaning all faucets, showers, and toilets.

Legionellosis 101

 Causes and Transmission

Workflow chart showing how Legionella affects building water systems and peopleLegionella bacteria are found naturally in freshwater and moist environments (e.g., lakes, rivers, groundwater) worldwide. Legionella can become a public health problem in manmade water systems, where it can grow and spread, causing disease. Most healthy people who are exposed to Legionella do not become sick. However, people aged 50 years or older, smokers, and those with underlying medical conditions such as chronic lung disease, or those with a weakened immune system, are at higher risk of becoming sick from Legionella or with legionellosis.

Outbreaks of Legionnaires’ disease are most commonly associated with large or complex water systems (e.g., hospitals, long-term care facilities, hotels, cruise ships).

People can become sick when they breathe in mist from a water source that has Legionella (water used for showering, cooling towers, decorative fountains, and hot tubs). This CDC infographic helps illustrate how Legionella is spread.

 Signs and Symptoms

Common signs and symptoms of legionellosis are:

  • Muscle aches
  • Headache
  • Tiredness
  • Loss of appetite
  • Coughing
  • Fever
  • Diarrhea (occasionally)

Contact your doctor if you develop symptoms, such as fever, cough, chills, or muscle aches. Be sure to mention if you spent any nights away from home in the last 10 days.


Treatment depends on whether or not the person has Pontiac fever or Legionnaires' disease. Pontiac fever requires supportive care only and will go away on its own. Legionnaires' disease is treated with antibiotics. Most people who get sick need care in a hospital but make a full recovery. However, about 1 out of 10 people who get Legionnaires’ disease die from the infection.


There are no vaccines against Legionella bacteria. To prevent Legionella bacteria from growing, it is important to make sure that water systems in buildings are being properly cared for and maintained.

CDC web resources on Legionella prevention:

 How Common is Legionellosis?

Legionnaires’ disease is on the rise in Wisconsin and nationally. The rising trend may be related to a combination of factors, such as increased awareness and testing, more people at risk (e.g., aging population, use of immune-suppressing medication), and increased Legionella in the environment.

Because Legionnaires’ disease is likely underdiagnosed, the number of cases may be underestimated. More illness is usually found in the summer and early fall, but it can happen any time of year.

Chart displaying Legionnaires' Disease cases in Wisconsin from 2010-2018


Provider Information

 Reporting and Surveillance Guidance

This is a Wisconsin disease surveillance category II disease:

Wisconsin case reporting and public health follow-up guidelines:

 Provider Resources

 Water Management Program Resources

In June 2017, the Centers for Medicare & Medicaid Services (CMS) released a survey and certification memo stating that healthcare facilities (hospitals and skilled nursing facilities) should develop and adhere to ASHRAE-compliant water management programs to reduce the risk for Legionella and other pathogens in their water systems.

The following resources may be useful when trying to understand what the requirement covers and for developing a comprehensive water management program.

Toolkits, Trainings, and Templates:

Fact Sheets and FAQ's:

Special Considerations:

Last Revised: October 15, 2021