Question: what differs between a good bacteria and a bad bacteria?

Lucy McGowan answered on 16 Jun 2020:

Good bacteria benefit from living inside us because it’s warm and there’s lots of nutrients for them. We benefit from them too; good bacteria help us to digest our food and help to keep bad bacteria away. Good bacteria also help to keep our immune systems healthy and strong. This is called a mutualistic relationship – meaning both humans and bacteria benefit from one another! Kind of like the old “You scratch my back, I scratch yours” saying.
Bad bacteria are parasitic, this means that they benefit from living inside us in the same way good bacteria do, but instead of helping our bodies, they damage them and make us feel poorly.