Step-by-step explanation:
So when u take antibiotics you kill lots of the bacteria. however you don't kill all of them. the bacteria that is left over hasn't been killed because they are resistant to the antibiotics taken. if left these bacteria will reproduce and build and species resistant to that type of antibiotics which can be incredibly dangerous. this is why it is necessary to finish your full antibiotics course so even if u feel better just to make sure you kill all the bacteria.