How Legionellosis Spreads?


Legionellosis is a disease caused by Legionella bacteria. There are two different types of Legionellosis:

  1. Legionnaires’ disease: This is the more severe form of infection that causes pneumonia.
  2. Pontiac fever: This is a mild respiratory illness without pneumonia.

Causes and Common Sources of Infection

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 building water systems like:

  • showerheads and sink faucets
  • cooling towers in air conditioning systems
  • hot tubs and whirlpools that aren’t drained after each use
  • decorative fountains and water features
  • hot water tanks and heaters
  • birthing pools
  • large plumbing systems
  • drinking fountains

Although it is possible for Legionella bacteria to grow in home plumbing systems, most outbreaks of the bacteria occur in large buildings with complex water systems.

How Legionellosis Spreads

After Legionella grows and multiplies in a building water system, water containing the bacteria spreads as droplets small enough for people to breathe in.

People can be infected by Legionella bacteria when they breathe in small droplets of water contaminated with the bacteria. Less commonly, people can get sick by aspiration of drinking water containing Legionella. This happens when water accidentally goes into the lungs while drinking. People at increased risk of aspiration include those with swallowing difficulties.

People at Increased Risk

Most healthy people exposed to Legionella do not get sick. People at increased risk of getting sick are:

  • people 50 years or older
  • current or former smokers
  • people with a chronic lung disease (like chronic obstructive pulmonary disease or emphysema)
  • people with weak immune systems or who take drugs that weaken the immune system (like after a transplant operation or chemotherapy)
  • people with cancer
  • people with underlying illnesses such as diabetes, kidney failure, or liver failure


Common symptoms of a Legionella infection include:

  • dry cough
  • high fever
  • chills
  • muscle aches
  • diarrhea
  • fatigue
  • headache
  • abdominal pain

Legionella bacterial infections are usually treated with Erythromycin or a combination of Erythromycin and Rifampin. There are no vaccines that can prevent Legionnaires’ disease.

How Legionellosis Spreads

Legionnaires’ disease

Legionnaires’ disease is a severe form of pneumonia, requiring treatment with antibiotics to kill the harmful Legionella bacteria in the lungs. It is commonly associated with water-based aerosols and often a result of poorly maintained air conditioning cooling towers and potable water systems. It occurs where water, contaminated with the legionella organism, is aerosolized and then breathed in by workers or patients. Legionnaires’ disease is not contagious but is a potential environmental hazard. Consequently, only those who are directly exposed to a contaminated aerosolized water source can get the disease.

Healthcare facilities often have large, complex building water systems. They frequently undergo construction and plumbing changes. They often have aerosol-producing devices, like cooling towers, decorative fountains, and other devices unique to healthcare facilities (e.g., respiratory therapy equipment, hydrotherapy tubs, heater-cooler units). Patients in these settings often have risk factors for Legionnaires’ disease, such as advanced age, weakened immune systems, and chronic medical conditions.

Most cases of this illness can be treated successfully. Healthy people usually get better after being sick with Legionnaires’ disease, but they often need care in the hospital.


Legionnaires’ disease usually develops two to 10 days after exposure to legionella bacteria. It frequently begins with the following signs and symptoms:

  • headache
  • muscle aches
  • fever that may be 104°F (40°C) or higher

By the second or third day, you’ll develop other signs and symptoms that can include:

  • cough, which might bring up mucus and sometimes blood
  • shortness of breath
  • chest pain
  • gastrointestinal symptoms, such as nausea, vomiting and diarrhea
  • confusion or other mental changes

Although Legionnaires’ disease primarily affects the lungs, it occasionally can cause infections in wounds and in other parts of the body, including the heart.

Possible complications of Legionnaires’ disease include:

  • respiratory failure
  • septic shock
  • kidney failure

When not treated promptly, Legionnaires’ disease can lead to death. About 1 out of every 10 people who gets sick with Legionnaires’ disease will die due to complications from their illness. For those who get Legionnaires’ disease during a stay in a healthcare facility, about 1 out of every 4 will die.

Pontiac Fever

Pontiac fever symptoms are primarily fever and muscle aches; it is a milder infection than Legionnaires’ disease and resembles the flu. Symptoms begin between a few hours to 3 days after being exposed to the bacteria and usually last less than a week. Pontiac fever is different from Legionnaires’ disease because someone with Pontiac fever does not develop pneumonia.


Pontiac fever can produce the following symptoms:

  • fever
  • chills
  • headache
  • muscle aches

Medical providers can use a urine or blood test to see if someone has Pontiac fever. However, a negative test doesn’t completely rule out that someone does not have the illness. Clinicians most often diagnose Pontiac fever when there are other known laboratory-confirmed Legionellosis cases (either Legionnaires’ disease or Pontiac fever) who may have been exposed to Legionella at the same time or place. Pontiac fever goes away without specific treatment.


Legionella grows best in warm, stagnant water, like the water temperatures used in hot tubs. However, warm temperatures also make it hard to keep disinfectants, such as chlorine, at the levels needed to kill germs like Legionella. Disinfectant and other chemical levels in hot tubs should be checked regularly and hot tubs should be cleaned as recommended by the manufacturer.

Water Management Programs

Avoiding water conditions that allow Legionella bacteria to grow to high levels is the best prevention.

Specific preventive measures include:

  • Regularly maintain and clean cooling towers and evaporative condensers to prevent Legionella growth. This should include twice-yearly cleaning and periodically using chlorine or another effective biocide.
  • Maintain domestic water heaters used in the workplace at 140°F (60°C) and maintain the water temperature at 122°F (50°C) or higher at the faucet.
  • Avoid conditions that allow water to stagnate. Unused waters lines should be frequently flushed to help alleviate stagnant water. Large water storage tanks that are exposed to sunlight can produce warm conditions that are favorable to high Legionella levels.

Medical Awareness of Physicians and Healthcare Workers

Healthcare workers need to be aware that the bacteria can be present in water systems promptly test vulnerable and/or symptomatic patients and use appropriate antibiotics quickly.

As mentioned earlier, Legionnaires’ disease most frequently attacks individuals who have an underlying illness or weakened immune system. The most susceptible include persons who are elderly, smokers, and individuals with suppressed immune systems.

Healthcare-Associated Outbreaks

How health department investigators respond to healthcare-associated cases and outbreaks of Legionnaires’ disease depends on the type and size of the healthcare facility, the existing capacity of the facility and health department, and the number of cases. Public health officials should work closely with healthcare facility staff at each step in the process. The appropriate healthcare facility points of contact (e.g., administrator, infection preventionist, clinician, quality assurance representative, facility manager, or engineer) may vary, depending upon the step.

Deciding Whom to Test for Legionnaires’ Disease

In healthcare settings, public health officials and healthcare providers should consider Legionnaires’ disease as a possible diagnosis in any patient at risk for Legionnaires’ disease with healthcare-associated pneumonia (pneumonia with onset ≥48 hours after admission). This is especially important among patients with severe pneumonia (e.g., those requiring intensive care) or if you identify any of the following in a healthcare facility:

  • other patients with healthcare-associated Legionnaires’ disease diagnosed in the past 12 months
  • positive environmental tests for legionella in the past 2 months
  • current changes in water quality that may lead to Legionella growth (e.g., low residual disinfectant levels, temperatures permissive to Legionella growth, nearby construction, areas of stagnation)
About Badar Javed

I have worked in the safety industry for more than 10 years, collaborating with different organizations to establish and supervise safety protocols. My expertise covers a broad spectrum, from construction sites to oil refineries, and I have personally witnessed how safety measures safeguard both employees and customers.

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