system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why are we required to finish a course of antibiotics?
gpt
Let's say your body is like the lawn in front of your house. 

You've got grass, and some bushes, maybe a tree or two. All of these are good things you want to keep around. But then some ugly weeds start growing in your yard and your dad wants to get rid of them.

He goes to the store and buys some weed killer and sprays it on the weeds for a day or two until the lawn looks better. But your dad didn't follow the directions, and now not only does he have half a bottle of weed killer left, some of the weeds didn't really die - it just seems like they did.

The weeds that survived, grow stronger, and move to your neighbor's yards. It takes stronger weed killer to affect them, sometimes strong enough to hurt the grass and bushes that you want to grow there. 

If your dad had followed the directions, and used the weed killer properly, his lawn would be healthy, and he wouldn't be ruining his neighbor's lawns too.

[Source: currently studying clinical microbiology]