Legionella bacteria are not new. While the earliest recorded samples of the bacteria date back to just 1947, and the earliest known cases of Legionnaires Disease dating back to 1965, this is most likely to be more of a reflection of the development of human microbiological technology and knowledge throughout the twentieth century. Legionella bacteria are naturally occurring, and can be discovered in low numbers in environments (water and soil) all over the world. As such, it can be expected that they have been with us for many thousands of years.

While the bacteria is naturally occurring, until the late 20^th century, it had not been a problem for humans. Throughout the 20^th Century, as technology advanced, humans designed systems which utilised water in a multitude of ways, and inadvertently gave this bacteria an opportunity to thrive within these systems. Due to the way that a number of these systems operate, we also gave the bacteria a platform to be disseminated into the environment, and subsequently, provided ourselves as potential hosts.

There are many different species of Legionella (at least 40), about half of this number are capable of causing diseases in humans. All of these illnesses are referred to using the general term ‘Legionellosis’.

Of these species, one causes about 90% of cases. This is known as Legionella pneumophila. The reason for the name Legionella, is because the first recognized outbreak occurred at a Convention of the American Legion, in 1976 (earlier dated cases have been retrospectively identified using stored lung tissue samples).

Legionella pneumophila can also be divided into sub groups. These sub groups are referred to as Sero-Groups. There are 16 different Sero-groups of Legionella pneumophila, and Serogroup 1 is associated with most cases of Legionnaires Disease (it is also associated with the majority of fatalities from Legionnaires Disease, but not all).

Humans contract Legionnaires Disease by inhaling water droplets, which contain Legionella bacteria, deep into their lungs. The bacteria then quickly colonise the lungs and after an incubation period of 2-10 days, symptoms develop. Often, the person exposed to the aerosol, would not even be aware of the presence of the aerosol. The bacteria can be transported by aerosols large enough to hold them. However, only those aerosols between 1 and 5 microns (0.001 – 0.005mm) in size can be inhaled sufficiently deep into the lungs.

Most healthy people, even when directly exposed to Legionella bacteria will be able to fight off an infection, however, a greatly increased risk surrounds the elderly, Immuno Compromised / Immuno Suppressed, Smokers, etc etc.

Legionella is known as the silent killer in some circles. The reasons for this are because a) people are generally unaware that they are being exposed to the bacteria, and b) the symptoms of a Legionella infection are very similar to many other illnesses / diseases which hold a far higher level of public awareness. Initial symptoms may include loss of energy, headache, nausea, aching muscles, high fever (often exceeding 40°C), and chest pains. Later, many bodily systems as well as the mind may be affected. The disease eventually will cause death if the body’s high fever and antibodies cannot defeat it. Victims who survive may suffer permanent physical or mental impairment.