No life exists "by themselves". Self-replication means using only its own DNA and not mangling with other's. Virii are not only parasites but dead matter (a ribonucl molecule surrounded by proteins that happens to stick to other cells, like dirt on the skin). Gut microbioma is parasite.
There is another life property that this object does not fulfill and is called Teleonomia, that is governed by an ultimate goal.
If you say "well not by themselves" neither do humans.