From publisher blurb:
Strange, alien life forms exist throughout the universe whether we know about their existence or not. Many superheroic settings assume the existence of such life forms. In some settings, only a handful of people have witnessed evidence of these creatures, let alone encountered one of them firsthand. In other settings—especially futuristic ones—aliens might walk and live openly among society. Depending on the nature of your setting, these creatures may seek a trade, a sharing of knowledge, or a chance to learn about humanity. Some of them, however, come to our world for conquest and destruction. Of the latter, the biological intruder represents an insidious and hidden threat.
A biological intruder is often a parasitic or symbiotic creature that needs a biological host to survive. The intruder might originate from distant worlds, other dimensions, or from greedy corporations. The hero or villain with the Biological Intruder template represents an individual who has, willingly or not, become host to one of these creatures and thereby gains new powers and abilities.