In what way do you see microbial flora being a benefit to us and in what way can they be a danger?

In what way do you see microbial flora being a benefit to us and in what way can they be a danger?
What would happen if we were to inadvertently kill off the resident flora that lives on and within us? Can you describe two or more diseases that are the result of killing our natural bacteria? How would you fix the problem?  200-250 WORDS