is the study of how diseases move around. There are three major factors that are considered in epidemiology. They are linked and often called the epidemiological triangle:


Understanding the links between these three factors allows epidemiologists to identify where interventions can be used to prevent disease. If we can control any one of the three then we can effectively control the disease. Once the triangle breaks then the disease cannot move around. So epidemiology helps us to find the weak link. Let’s look at these three factors and how they relate to Legionnaire’s disease.


Legionella are common but not abundant in the environment. That is, they are often found but usually in low numbers, especially in fresh water.

Legionnaire’s disease is a non-transmissible Legionella infection. There is only one documented case of person to person transmission. The bacteria cause a serious pneumonia. This is known as an opportunist infection, causing disease by a case of mistaken identity. In the natural environment protozoa, algae and possibly some worms are the natural hosts.

Legionella can infect protozoa that would normally digest them. In a reversal of roles Legionella, once ingested, go about digesting the protozoa and multiplying inside them. The case of mistaken identity occurs when human lung phagocyte ingest Legionella. Phagocytes are protozoan–like cells that move around the lungs digesting bacteria and debris that may enter during normal breathing. In the immune compromised person these cells are less able to detect foreign matter and become vulnerable to Legionella attack.


The human host is a person whose immune system is compromised permitting the Legionella to multiply. Healthy individuals would fight off the infection quite easily. Disease is caused by entry of contaminated water into the lungs. This may be by inhalation of fine water droplets (aerosol) or by aspiration (accidentally sucking water into the lungs). High risk factors for disease are persons over 50 years, smoking, deliberately immune compromised persons (eg transplant recipients) and those with a previous history of respiratory disease. Unaccountably the disease affects roughly 3:1 males to females.


Most Legionella are aquatic and live in lakes, rivers, hot springs, and moist soil. The problem begins when they enter artificial water systems. For some reason once inside building water systems Legionella multiply to much higher concentrations. Much higher than would normally occur in their natural environment. Once inside buildings their growth in biofilms affords protection from disinfection. Most Legionella are thermally tolerant and will cope with water temperatures up to 50⁰C of more.

The bacteria are distributed to susceptible hosts once they are established in the building water systems. This may be in aerosol from things like cooling towers, spa pools or showers. Aspiration from the potable water supply eg tap water or contaminated ice also occurs.

Putting it Together

Our modern built environment provides new niches where Legionella can multiply. Quite often buildings combine large surface areas for biofilm attachment and temperatures that favour growth. This makes them ideal targets for Legionella colonisation.

Usually hospitals, aged care premises and nursing homes have complex water systems where Legionella can grow. These places also usually contain large numbers of the susceptible host population.


Legionella is highly likely to persist in the natural environment. So it is highly likely it will continue to enter the built environment. As medical innovations continue so will the rising percentage of our population that are a high risk for Legionella infection. Consequently the HOST and the AGENT appear to be here to stay. This leaves the ENVIRONMENT. Implementing effective water treatment and system controls can minimise the risk of Legionella infections. In fact several source state it is an ‘entirely preventable disease’. In conclusion a Water safety and risk management plan is the most effective way to break the epidemiological triangle.

Further Reading

World Health Organisation (2007) Legionella and Prevention of Legionellosis

World Health Organisation (2011) Water Safety in Buildings