The intimate relationships of symbiosis are looking more and more important in our understanding of how living things interrelate, and how certain important evolutionary events occurred. We recognize three basic types of symbiosis, though of course symbiotic relationships really represent a spectrum from one extreme to the other, with every intermediate imaginable present at least somewhere. It's a colony of a number of different organisms involved in a set of complex and mutually beneficial symbiotic relationships. E. coli bacteria can be parasites if they get into your gut.
Viruses are parasites, though they are only sort of alive.
This is called endosymbiosis, and its implications for evolution are very profound. In fact, consider the cells of all organisms except bacteria. Besides the nucleus, eukaryotic cells contain many, many other structures inside themselves. One of those structures, present in many copies in every eukaryotic cell, is the mitochondrion. There is excellent evidence that, about 2.5 to 3 billion years ago, mitochondria came into existence as endosymbiotic bacteria, living entirely inside the cells of another organism.