Thought it had to be the 'fumes' from stagnant water kept at a highish
temp and breathed in a confined space? Like from water cooled air con?

Nah, it often comes through the DHW supply and out of the tap or shower.

However, air conditioning condensors are often in the 20C-45C critical range
as well, so air conditioning is a major player, too, particularly in the
larger outbreaks.

Christian.