some microorganisms have a positive impact on human body & if something has a positive impact on another thing, then the relationship is benificial ->  some microorganisms are benificial to human body  if something has a negative impact on another thing, then the relationship is harmful & disease-causing microbes have a negative impact on the body ->  some disease-causing microbes are harmful to human body  some microorganisms are benificial to human body & some disease-causing microbes are harmful to human body -> the relationship between human and microorganisms is sometimes beneficial and sometimes harmful