Legionella is a genus (i.e. group) of Gram-negative pathogenic bacteria (which includes L. pneumophila) which are known to cause a pneumonia-like illness known as legionellosis or Legionnaires' disease, and a mild flu-like illness called Pontiac fever.

Legionella is a naturally occuring bacteria that is widespread in water or soil but rarely cause infections however, in complex water systems made by humans (e.g. hospital plumbing systems, cooling towers) Legionella can rapidly multiply and when inhaled may cause Legionnaires' disease.

Whilst in theory it is possible to contract Legionnaires' disease from home plumbing, generally most cases result from more complex systems such as those that occur in large buildings, hospitals or cooling towers. This is likely because these complex systems allow the bacteria to grow and spread more easily / rapidly (3).



Legionella sp. colonies growing on an agar plate and illuminated using ultraviolet light. Image courtesy of CDC / James Gathany (1)


Legionella was given its name after an outbreak of the then unknown bacteria cause illness in 221 persons and cause the death of 34 people at a convention of the American Legion in Philadelphia in July 21 – 24 1976. On January 18, 1977 the causative agent (then uknown) was identified as a bacterium subsequently named Legionella (2).

Legionella does not spread from person to person, but rather a person typically becomes infectegd when they inhale a microscopic water droplet which contains the bacteria. It can often be from the spray of a shower, tap or even water aerosols from a nearby cooling tower (3).

In addition to infection by inhalation of water droplets, people have also obtained Legionnaires' disease through aspiration (when liquids accidently enter your lungs during drinking) of liquid which contains the bacteria, or in working in a garden or with contaminated soil / potting mix (3).

Generally speaking, Legionnaires' disease most commonly occurs in the warmer months in Australia. Whilst anyone can become infected, the disease is more common in middle aged and older people as well as those whose immune system is weak. Interestingly men are typically more frequently affected than women. The symptoms of Legionnaires' disease include (4);

  • Fever
  • Cough
  • Chest Pain
  • Diorrhoea
  • Breathlessness

The risk of infection (given a source) is increased by (4);

  • Smoking
  • Chronic heart or lung disease
  • Diabetes
  • Kidney Failure
  • Some forms of Cancer
  • Immunosupression (more common if on steriod medication)
  • Being 50 years or older

The incubation period is usually 2 to 10 days from becoming infected and developing symptoms, it may be longer but as earlier noted the disease is not infectious (4). The disease can be very serious and has caused death in some people.

Stay tuned to the blog as I will likely do a bit more on Legionella and its incredible adaption around macrophages.


References & Attribution

  1. Image: Legionella sp. colonies growing on an en:agar plate and illuminated using ultraviolet light to increase contrast, 2005, CDC Public Health Image Library (ID#: 7925), CDC/James Gathany
  2. Lawrence K. Altman (August 1, 2006). “In Philadelphia 30 Years Ago, an Eruption of Illness and Fear”New York Times.
  3. https://www.mayoclinic.org/diseases-conditions/legionnaires-disease/symptoms-causes/syc-20351747
  4. https://www.sahealth.sa.gov.au/wps/wcm/connect/public+content/sa+health+internet/health+topics/health+conditions+prevention+and+treatment/infectious+diseases/legionella+pneumophila+infection/legionella+pneumophila+infection+-+including+symptoms+treatment+and+prevention
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