When cleaning your shower head to prevent the Legionnaires disease take the shower head off the pipe and then place the shower head in a mixture of baking soda, white vinegar and hot water.
Allow the shower head to soak in the mixture 2 hours to overnight.
The longer you allow the shower head to soak in the mixture of baking soda, vinegar and water the more bacteria will be killed and the shower head will be cleaned.
Then rinse the shower head off after soaking it.
Then when you put the shower head back on the shower pipe run hot water through the shower head and it should clean it out.
Do this every 6 months or so and also put a bag filled with vinegar on the shower head every few weeks or so to help kill bacteria.