This is the bacteria that first came to mind yesterday when reading this thread..
https://en.wikipedia.org/wiki/Legionnaires%27_disease
L. pneumophila thrives in aquatic systems where it is established within amoebae in a symbiotic relationship.[23] In the built environment, central air conditioning systems in office buildings, hotels, and hospitals are sources of contaminated water.[21] Other places it can dwell include cooling towers used in industrial cooling systems, evaporative coolers, nebulizers, humidifiers, whirlpool spas, hot water systems, showers, windshield washers, fountains, room-air humidifiers, ice-making machines, and misting systems typically found in grocery-store produce sections.[16][24]
The disease may also be transmitted from contaminated aerosols generated in hot tubs if the disinfection and maintenance program is not followed rigorously.[25] Freshwater ponds, creeks, and ornamental fountains are potential sources of Legionella.[26] The disease is particularly associated with hotels, fountains, cruise ships, and hospitals with complex potable water systems and cooling systems. Respiratory care devices such as humidifiers and nebulizers used with contaminated tap water may contain Legionella species, so using sterile water is very important.[27] Other sources include exposure to potting mix and compost.[28]
Legionella bacteria survive in water as intracellular parasites of water-dwelling protozoae, such as amoebae. Amoebae are often part of biofilms, and once Legionella and infected amoebae are protected within a biofilm, they are particularly difficult to destroy.[16]
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I don't know what the risk factor would be in anyone's set up, perhaps very minimal, but personally I tend to avoid potential issues such as this whenever possible.
