From Introduction:
Mutated viruses capable of transforming any unfortunate enough to be infected into utterly inhuman-and often monstrous-entities; clouds of alien spores, possessing a weird form of collective intelligence, that seek out suitable hosts as means of reproduction; nanites-self-replicating robots on the molecular scale-programmed to commandeer the DNA of organic life forms; all of these are examples of biological intruders. Biological intruders are malignant microorganisms that infect and seize control of another creature, multiplying inside the host body until the infected being’s biological processes are completely replaced by that of the biological intruder. Unlike most other templates, a character who becomes host to a biological intruder is less changed than infected; unlike symbiotic organisms, which adapt to functioning with the host’s body, biological intruders are deadly opportunistic organisms, using other living beings only as means of maturing or reproducing, and effectively slaying them in the process. A biological intruder that is allowed to progress to its’ final stages ravages the character’s physical and mental faculties, eventually destroying the host’s mind and vital functions and replacing them with its’ own.
