Organisms that live on other organisms and do not give anything in return are known as

At most times, these living organisms called parasites, occupy any living object, and kill it slowly, they cause great sickness for humans and other animals, and usually you don't want to get infected with one. It's worse than having the flu if you get a bad one!

Parasites.

Certain types of organisms will live on other organisms but will give something in return, but organisms that do not give anything in return are called parasites.