For reproduction purposes, many parasites require a specific host to reproduce in. An interesting example is a worm that mind controls a snail and gets itself eaten by a bird, and then reproduces in the bird. Surprisingly, both the snail and the bird survive this process. (Granted, the difference between this and a virus is the virus uses the RNA decoding infrastructure in the infected cell to reproduce itself, while a parasite just is adapted to reproducing in the environment of the hosts body, but uses its own cells to do the reproduction).

However, there are many, many examples in nature of some essential task (often some part of the energy production/absorption process) that are done by a different organism. Some particularly interesting examples:

• there are a handful of animals that eat plants, absorb the chloroplasts, and use those to do photosynthesis

• In most animals, even in humans, a lot of the digestion process is done by bacteria living in your digestive tract. Some illnesses are caused by issues with the digestive tract bacteria, such as them dying out.

There are other animals adapted to living in environments or using things produced by other organisms. Hermit crabs get their name from their behavior of borrowing shells created by other organisms.

Really the only organism that can truly live “by itself” would probably be something like algae.