Credit: CDC/James Gathany

Legionella is a type of bacteria which causes Legionnaires’ disease, a form of pneumonia which is a major threat to those vulnerable due to either age, illness, immunosuppression, smoking or a combination of these.  Legionnaires’ disease becomes infectious when inhaled through airborne droplets or particles that contain legionella bacteria, which is small enough to penetrate deep into the lungs and be deposited on the alveoli.  There other types of disease associated with legionella however these have been given a generic term of legionellosis.

To date 37 different species of legionella bacteria have been found. The species that is associated with disease outbreaks of Legionnaires’ disease is L.pneumophila which is widely found in small amounts in natural water sources. However the bacteria is not a risk until it comes into contact with devices like air conditioning units or hot and cold water services, which can provide the right conditions for multiplication and dissemination of the bacteria.

At present it is still unclear as to what dosage of legionella is necessary for a human being to become infected. Research into minimising the risk of the disease is ongoing.