An organism that grows on or in other organisms is called a parasite.

Here's a breakdown:

* Parasite: An organism that lives in or on another organism (the host) and benefits by deriving nutrients at the host's expense.

* Host: The organism that is being lived in or on by the parasite.

There are many different types of parasites, including:

* Ectoparasites: Live on the outside of the host's body, such as fleas, ticks, and lice.

* Endoparasites: Live inside the host's body, such as tapeworms, roundworms, and bacteria.

Parasitism is a common relationship in the natural world, and it can have a significant impact on both the parasite and the host.