Rainwater batch heaters and pathogens

If I built a small, rainwater collection system that fed directly into a batch heater that might sit full of rainwater for a few days before taking a shower, being heated by the sun, can anyone think of problems with pathogens that might grow? I read the thread on Legionnaires disease and still am not clear on wheter it is in rain water. Any help? Thanks, Morgan.

Sunlight prevents its growth, and airation of the water does as well. Also if the water is above 120f or under 60f

from Treehugger.com

Legionnaires Disease, or Legeionellosis, is caused by "Legionella pneumophila, a ubiquitous aquatic organism that thrives in warm environments." It was identified after 34 veterans died after attending an American Legion Convention in the Bellevue-Stratford Hotel in Philadelphia in 1976. It is temperature sensitive:

    * 70 to 80 °C (158 to 176 °F): Disinfection range
    * At 66 °C (151 °F): Legionellae die within 2 minutes
    * At 60 °C (140 °F): Legionellae die within 32 minutes
    * At 55 °C (131 °F): Legionellae die within 5 to 6 hours
    * Above 50 °C (122 °F): They can survive but do not multiply
    * 35 to 46 °C (95 to 115 °F): Ideal growth range
    * 20 to 50 °C (68 to 122 °F): Legionellae growth range
    * Below 20 °C (68 °F): Legionellae can survive but are dormant

I have a 55 gallon drun in the grenhouse which if filled with water.  i pump it through a loop layin gin the sun, back to the tank.  The tanks typically heats up to 120 degrees.  The top is open and the water is not cycled or changed out.  I get some growth in there, mostly on the bottom.

UV radiation can destroy bacteria in just a couple of hours.  The depth of water that the sunlight must penetrate will reduce the UV rays that reach the core of a tank.  A shallow pan with large surface area covered with glass can go a long way to destroying bacteria.