For as long as humans have walked the earth, we’ve been accompanied by microscopic beings that are part of our bodies but not ourselves. The viruses and bacteria that coexist with humans have shaped who we are, but that doesn’t mean they are all benevolent.
http://www.fredhutch.org/en/news/center-news/2017/03/lessons-learned-about-cancer-causing-bacteria-viruses.html
What we’re learning about cancer-causing bacteria and viruses