Symbiotic relationships are very common in nature.  Such relationships are either synergistic, anatagonistic or neutral, depending upon the amount of cooperation between members (symbiotes).  In mutualism, two or more species cooperate to the benefit of both, as seen in termites and their cellulose-degrading protozoan and bacterial gut inhabitants (above) and the resident microflora in and on the human body.

Commensualism occurs when two or more organisms interact such that one benefits while not having any significant positive or negative impact on the other.  Examples of this include the relationships between large predatory sharks and remoras (fish that attach themselves to the body of the shark) and the resident microflora of human skin such as staphylococci, fungi, nonpathogenic mycobacteria and corynebacteria.

Parasitism occurs when two or more species interact in an antagonistic fashion, where one species (the parasite) benefits to the detriment of the other (the host).  Viruses, parasitic worms and insects, pathogenic fungi, protists and bacteria are all parasites that exploit host resources for their own benefit.