@rhodges
Swamp coolers have been used for many decades, particularly in commercial water cooling towers. It is good you haven't gotten sick from Legionella, but its incidence has been increasing. Northerly areas seem more susceptible than southerly climates.
When people talk about microbial pathogens, they sometimes lump everything together as "germs." Not all bacteria are the same, nor do they present the same potential for causing pneumonia. Legionella was first identified in 1976. After that, cases dating back to 1943 were found. It may well go back further but was only considered a type of pneumonia then. Pneumonia is a clinical diagnosis. In most cases, an etiologic agent is not identified.
One can isolate a lot of microorganisms from water. Most are harmless as respiratory pathogens in otherwise immuncompetent people, but times have changed. As one example, after WWII, our military dispersed a pigmented Serratia marcescens over San Francisco as a surrogate for radioactive particles to trace fallout patterns. Because its colonies were pigmented, it was easy to track with simple cultures. In those days, it was considered completely non-pathogenic; today, that is not the case.
I would always be concerned about a swamp cooler, particularly if it were left running for long periods without decontamination.