There is a mix in the area that have helped create your biome. Now go into an area with decomposing bodies. The additional bacteria load will overwhelm most.

There is a mix in the area that have helped create your biome. Now go into an area with decomposing bodies. The additional bacteria load will overwhelm most. It's a little more complex qnd the way you explain it is how I would introduce the topic to my child. You may have billions of bacteria but if you introduce billions of different types you may not have the bacteriophages to deal with those types and your body may produce these as a defense (or not if your immune system is weak) but what happens in many cases your body may kill off beneficial bacteria and that gets replaced with a similiar but non-beneficial bacteria and because the phages changed you can't get back. We see this with fluoroquinolone antibiotics[1] Davies EV, Winstanley C, Fothergill JL, James CE. The role of temperate bacteriophages in bacterial infection. FEMS Microbiol Lett. 2016;363(5):fnw015. doi: 10.1093/femsle/fnw015