This is inside an indivudal who has a bacterial infection and was just prescribed antibiotics
The antibiotics killed some of the bacteria but some are still left alive and only exposed to the antibiotic. These bacteria are more likely to have a mutation to be resistant to this antibiotic
These bacteria evolved and are now resistant to that antibiotics. So now they have a selective advantage and can reproduce.
If this over-prescription continues, the bacteria will continue to become resistant until there isn't a antibiotic that can kill this strain of bacteria
Now by natural selection and over prescribing antibiotics, this individual is sick again and needs stronger antibiotics to become kill these bacteria.
