Thanks to Ian Johnston for his thought-provoking post. The clever beavers' dams change the local ecology, favoring some plants and animals, harming others.
Rain is good, but lots of rain causes destructive flooding.
A similar spectrum is there for microbes in our bodies. In discussing lethal microbes with your students at Bethel College, I'd recommend teaching them some of the biological processes which these microbes take advantage of. Cell membrane traffic regulation and gene transcription are totally cool and awe-inspiring processes. The students should learn that there are all manner of microbes ranging from nice to naughty and everything in between.