I've just finished a training course on Legionella.....
If you decide to make use a swamp cooler, consider Legionnaire's disease. It's caught from breathing in fine water mist carrying the bacteria and these devices can be a perfect breeding ground, if you're not careful.
Depending of the strain of bacteria, the fatality rates are 12%. It presents as pneumonia/serious flu (more serious than man-flu if you can believe that!)

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The bacteria thrives in:
Water temperatures between 20C-45C 68f-113f
Water with nutrients like algea and sludge at the bottom of a tank to feed on
A rusty or slimy tank walls/bottom to hang onto
Keep it clean and replace the water regularly keeps the risks low. It's worth sterilising the hangy down mats/pads used for evaporation once in a while.
The bacteria naturally occur in rivers dams and streams so I'd advise you to sterilize the water before using it, if that is your source. The bacteria is killed above 60C so boiling the collected water would probably do it. Purifying tablets are another way. In some places where it's really hot an sunny; by leaving river/dam water in a clear plastic "coke" bottle in the sun all day has a double effect of UV and heat killing the bacteria.
http://news.bbc.co.uk/1/hi/world/africa/4786216.stmDepending on the part of the world you live in, your drinking water from the tap is normally free from Legionella.
Hot tubs/Jacuzzi unless maintained, drained and cleaned are ideal breading grounds too.....
To put this in perspective in the uk (pop 60 million) 12 people a year die from Legionnaires disease and of those 50% catch it when out of the country.
Caveat:
Free advice and worth the price you pay. Tim McGraw.
For legit guidance please download the free version (little link near the buy it now button)
http://www.hse.gov.uk/pubns/books/l8.htm