[splatterdash]

Almost every part of the human body directly exposed to the environment is covered with microbes: the skin, the gut lining, your mouth, your nasal passage, etc. Only a small part isn't (as I recall your eyes are sterile). I don't have proper citation for this, but AFAIK if you disrupt the microbial balance on these surfaces it might result in health-related problems. So in a way, yes, we need them to live healthy.

Other parts have to be kept sterile. Your brain, your heart, your lymph nodes, are some examples. When these parts gets infected, you get sick.

On another note, microbes are fantastic creatures. The earth is covered in them and they've been around much longer than us. They can live off a myriad of different substances and even without the sun.

Yay microbes :p!