Question: Why are some bacteria good and some bad?

Hayley Clissold answered on 17 Jun 2015:

All types of bacteria (either good or bad) are just trying to survive and reproduce to keep their species alive. Thankfully, most bacteria are able to do this in a way that either has no effect on us or is actually beneficial to us (e.g. keeping our digestive system working, helping us to make food products etc). Unfortunately some pathogenic bacteria are only able to survive and reproduce in a way that is disadvantageous to us (e.g. living inside our cells and then breaking free to release their progeny, which may cause us harm). It’s a constant battle for survival, really.