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  Legionnaires' disease

Legionnaires' disease

Was named after an outbreak of a severe pneumonia-like disease that occurred at the Bellevue Stratford Hotel, Philadelphia, USA in 1976, where there was a convention of the American Legionnaire veterans.

It is a form of pneumonia caused by bacteria of the genus Legionella.

It is still not known how many Legionella bacteria (CFU or GU) are required to contract the disease.

Legionnaires' disease is a potentially fatal form of pneumonia and everyone is susceptible to infection.

Risk increases with age, but some people are more susceptible i.e. people over 40, smokers and heavy drinkers, people suffering from chronic respiratory or kidney disease, diabetes, lung and heart disease or anyone with an immunocompromised situation.

The epidemiologist who isolated the bacteria was Mr Robert Sharrar, who has gained little recognition from growing the organism.

People catch Legionnaires' disease usually by inhaling aerosols (not droplets of water) suspended in the air, which contain the bacteria. The other infection route is by aspiration i.e. via ingesting a fluid which enters the windpipe by mistake.


Legionellosis is any illness caused by exposure to Legionella.

Other manifestations of Legionnaires' disease, with less associated mortality are Pontiac fever and Lochgoilhead fever.

Legionella pneumophila and other related bacteria are common in natural water sources such as rivers, lakes and reservoirs.

Usually found in low numbers, they may also be found in purpose-built water systems, such as cooling towers, evaporative condensers, hot and cold water systems and spa pools.

If conditions are favourable, the bacteria may multiply, increasing the risks of Legionnaires' disease.

It is therefore important to control the risks by introducing appropriate measures.


It is an erroneous belief that water droplets are an aerosol and that Legionella, or other bacteria, virus, etc. have to be contained within a wet droplet.

Water droplets may/can create the structure of an aerosol if the droplets are small enough.

Water evaporates from small droplets very speedily, and can occur in milliseconds.

For particles that are less than 5 micrometres, the rate of evaporation may reply on the temperature of the location, relative humidity and airflow.

An aerosol is defined as a suspension in a gaseous medium of solid particles, liquid particles having a negligible falling velocity. It is a suspension of particles which may contain legionella with a typical droplet size of < 5 micrometres that can be inhaled into the lungs.

Did you know that: “pneumophila” means lung-loving in Greek