Despite some of the hype from the media, legionella bacteria CANNOT be spread in a domestic air conditioning setting. The reported cases of legionnaires associated in cooling towers linked to commercial air conditioning systems is also extremely low due to strict regulations that are enforced in each state in Australia. This includes regular testing and biocide treatment under workplace health and safety laws.
In reality there is much more of a chance contracting the disease through breathing in particles in potting mix than through an air conditioning systems, and not all people exposed to the bacteria will be affected.
We aren’t downplaying the potential severity of the bacteria but it’s not realistic to suggest that commercial air conditioning has caused a spike in cases. There is also no concrete evidence that evaporative coolers are linked to spreading the bacteria. Features such as dump valves on modern commercial air conditioning systems to further lessen the risk.