Shower heads can have bacteria on them even without you knowing.
If you keep your shower head clean and disinfected then you should not have to worry about bacteria.
You can clean the shower head often with diluted vinegar and also diluted bleach.
The diluted bleach and diluted vinegar will keep the shower head clean and kill bacteria that can be on the shower head.
It's rare for someone to get sick from shower heads but it's possible to get sick and ill in rare cases from a bacteria laden shower head.
I disinfect my shower head at least once per week and keep my shower clean to prevent mold and other bacteria growing in the shower.
As long as you clean the shower head and shower often enough and disinfect the shower and shower head you should not get sick from the shower or shower head.