A game with this tag involves some kind of symbiotic or parasitic relationship with creatures in the world. It is usually sexual in nature, but it does not have to involve sex. The relationship can be consensual or forced, bilateral or unilateral, as the names "symbiosis" or "parasitism" suggest.

It is similar to a possession tag, with the major difference being that it involves some kind of physical, biological organism, rather than an incorporeal spirit, even if the parasite involved is considered to be demonic or fantastic.

The parasite or symbiote may cause drastic changes in the body and mind of the character involved, either permanently or temporarily. For example, it could include the addition of tentacles or covering the body in the form of living clothes. The characters may or may not be able to control the changes or the additional parts.

The effects may cause the character to become sexually active, lose their minds, or be controlled by the creatures. It is possible that the parasite or symbiote could give the characters supernatural or extraordinary powers and abilities at a price.