Bad bacteria (not a virus) make you sick -- sore throat, fever, earache... whatever.
Your doctor prescribes antibiotics. The medicine starts killing the bacteria, starting with the weakest cells. It can't really tell bad germs from your good germ-fighting cells, so it kills off some of those, too.
You start to feel better after taking your medicine for a few days and you forget to take it.
Oh no! The bigger, stronger bacteria have not been killed yet by the antibiotics! It only takes 20 minutes to a few hours for bacteria to double, triple, quadruple...
It's the STRONGER bacteria that are taking over!
And now your medicine won't work against the more numerous, stronger bacteria.