How have antibiotics been misused in the past?

1. Prescribed for diseases that aren't bacterial
2. Put into animal feed for animals that aren't sick

Many antibiotics work by destroying bacteria's ability to reproduce. Because this method takes time to work, those who were not prescribed enough or stopped taking it mid-script allowed some bacteria to survive. They then mutated to become resistant to the medicine's mechanism of action. That's why it's so important to take them completely until the bottle is empty.