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. We classify symbiotic relationships based on the harm/help consequences to the partners involved. While we always couch our definitions and explanations in terms of relationships between two species, there are many symbiotic relationships involving multiple species for example, the Portuguese Man-'O-War is not an organism. It's a colony of a number of different organisms involved in a set of complex and mutually beneficial symbiotic relationships. This kind of situation leads us to seriously consider the meaning of "organism."
At one extreme of the symbiotic scale is parasitism, where one partner (the parasite) benefits from the relationship and the other partner (the host) is damaged. A tapeworm is a parasite; you may be its host. E. coli bacteria can be parasites if they get into your gut. Viruses are parasites, though they are only sort of alive.
At the midpoint of the scale, we have commensalism, in which one partner (the commensal) benefits, and the other partner (again, the host) is indifferent.
At the other extreme we have mutualism, in which both of the partners (mutualists) benefit from the relationship. Our friend the Portuguese Man-'O-War is a mass of mutualists.
In all three types, and all of the uncounted "in-betweens," one of the partners may very well live entirely inside the other partner. This is called endosymbiosis, and its implications for evolution are very profound. For example, consider the cells of your body and your dog's, and your geranium's, and those of the athlete's foot fungus decorating your toes. In fact, consider the cells of all organisms except bacteria. These are what we call "eukaryotic" cells. That means cells with nuclei; bacteria have prokaryotic cells, which don't have any structure called a nucleus. 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. Mitochondria are very important to cells they are the energy converters that extract useful energy out of food. 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. The same goes for chloroplasts, those green photosynthetic structures in plant cells to which we owe so much.
One final evolutionary note about symbiosis. Symbiotic relationships show a very clear and easily understood evolutionary pattern. It is to the advantage of both a parasite and its host that the parasite damage the host as little as possible. Thus, the selective pressure is strongly in favor of decreasing harm to the host. Any accidental change in the parasite which allows it to keep its host healthier is favored; any accidental change in the host which protects it better from damage from its parasite is also favored. Therefore, parasitic relationships gradually evolve into commensalistic relationships, which gradually evolve into mutualistic relationships. Parasitologists (people who study symbiotic relationships, particularly parasitic ones) have recognized this for a long time. In general, they judge the length of time a particular parasitic relationship has existed by the amount of damage the parasite does to the host. Remember Ebola virus? Ebola is a very bad parasite, at least as a human parasite. It almost always kills its host very rapidly. The consensus among parasitologists is pretty much universal that Ebola is a virus which has only recently jumped hosts. In other words, it's a virus which had only recently "learned" to infect humans. ("Learned" in this case means that a chance genetic change in some members of the virus allowed it to broaden its host specificity to include humans.) When we find other species which can host Ebola, they are not nearly as devastated by it as we are. Conclusion: the relationship between those species and the virus is much older than the relationship between humans and the virus.
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