A good way to clean and disinfect your shower head is to soak the shower head in a bag of vinegar.
Pour a cup of distilled white vinegar into a large plastic food storage bag and then tie the plastic bag to the shower head and let it soak overnight.
Then the next day remove the bag and run the hot water to clean out the shower head.
The vinegar will not only clean and disinfect the shower head but it will also help remove calcium deposits from your shower head as well.
You can also disinfect the shower head by soaking the shower head in a bag of diluted bleach and the bleach will clean and disinfect the shower head as well.
Then run the hot water through the shower head to wash the bleach out and then wipe the bleach off the shower head.