Step-by-step explanation:
Okay the question is a bit confusing to understand but I believe you want to know about antibiotic resistance.
Let's say the child is sick and has started antibiotics, the antibiotics would work against the bacteria hence killing them so the child can recover, however mutation of the bacteria occurs and some were able to survive against the antibiotics.
Now it's like natural selection, only the bacteria that got mutated survived while the rest died, this gave the mutated bacteria an advantageous gene. This bacteria will then reproduce and pass on this advantageous gene causing the antibiotics administration to be ineffective, hence all the new bacteria is immune and there is an antibiotic resistance