Why do some microbes cause illness, while others don’t?
Jeannine Durdik, professor of biological sciences in the J. William Fulbright College of Arts and Sciences, replies:
There are lots of microbes on our planet and very few of them cause illness. Of course we hear the most about these troublesome ones. Those that cause illness have factors that distinguish them from the non-illness causing ones. We say these benign ones are not pathogenic. Some of the pathogenic ones often have sugars and proteins that allow them to gain deeper entry into our tissues. They are probably just seeking out the rich nutrients we contain.
Microbes and mammals have been co-evolving for a long time and the most successful ones even do us services — like helping us digest foods or protecting us from those that are harmful by taking up the spaces on our skin where otherwise a pathogen might get a foothold.