What is Legionella?

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  • Written By: Michael Pollick
  • Edited By: Niki Foster
  • Last Modified Date: 10 October 2019
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In 1976, 29 people attending a Legionnaire's convention in Philadelphia died from a mysterious illness. After months of investigation, the root cause of their deaths was traced to a bacterium called legionella, or more precisely Legionella pneumophila. The bacteria thrives in standing water sources such as ponds, wells, cooling towers, air conditioner ducts, and hot tubs. Although the original source of the illness was thought to be stagnant water collecting under air conditioning units, further testing suggested that this was not the main contributor.

Legionella derives its scientific name from the association with the Legionnaire's outbreak. Many people have heard of Legionnaire's disease, but the same bacteria is also responsible for a similar respiratory infection called Pontiac fever. Both illnesses are distinguished by their severe flu-like symptoms and gastrointestinal distress.

People cannot contract legionella through the act of drinking contaminated water or through skin contact alone. It's spread as an aerosol, meaning the bacteria is contained in small water droplets. If the infected mist can get past the throat's natural choking mechanism, it can be inhaled into the lungs.


Once the bacteria enters the lungs, it begins to feed on living tissue. The body sends out white blood cells to counteract the invading bacteria, but the result is often a growing infection and excessive mucus. Legionella pneumophila, which is responsible for 90% of all legionella infections, does not always respond well to penicillin-based antibiotics, which means the patient is often given a higher strength antibiotic cocktail to kill off the bacteria. If the patient is a heavy smoker or has a compromised respiratory system, the infection could become fatal. Many of the original 29 victims in 1976 were heavy smokers.

Legionella bacteria are known disease agents, so municipal water supplies are routinely tested for their presence. Many people exposed to it never develop any symptoms of Legionnaire's disease because contaminated water must be breathed into the lungs, not swallowed, in order to cause illness. Heavy smokers are more at risk because their choking mechanism doesn't work as well, leaving their lungs more vulnerable to the infected droplets of water. Some experts suggest limiting exposure to hot tubs and misting towers in order to avoid exposure to this bacteria and other aerosol-delivered infective agents.


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