Every once in a while, animals get sick and need to be treated in order to get well and give products again.
 
 
The treatment will cost some coins, but once you pay for it, you will get some energy in exchange, and your animal will be healthy again and able to give you resources!
 
Note: the game shows you how many more times you can get products from an animal before it gets sick. This allows you to plan your resources ahead.