If the water is scalding to the hand turn it down as it will be too hot for skin like the back. From the internet:-
Most experts recommend that hot water temperature should not exceed 125° F (51.66° C). A water temperature exceeding this poses serious risk of bad burns, particularly to children. In fact even at 125° F, if the child puts his or her hand in the water continuously for two minutes he or she may get second or third degree burns.
From a health point of view its a closed system so should be bug free, but make sure you keep your shower heads clean.