Leech/biology

Progenitor leeches
These leeches grow to approximately 20 centimeters in length and can be distinguished by the prominent dorsal organ, the function of which is unknown. On their own, they are more of a nuisance than a threat, although they tend to congregate in large numbers. They appear to spawn from large egg clusters, some of which can be seen aboard the Ecliptic Express. Progenitor leeches infest an area after the destruction of a Mimicry Marcus, from which they are composed. Like Mimicry Marcus, they are vulnerable to fire, although individually they have little health.

Mimicry Marcus
These humanlike forms are aggregates of Progenitor leeches assembled into a cooperative whole. They secrete an adhesive bodily fluid that allows them to maintain their collective shape. While stationary, they can mimic precise shapes and textures, allowing them to replicate the appearance of James Marcus almost perfectly; however, while moving, they appear formless, elastic, and highly flexible. Sufficient damage will force them to abandon their shape for the sake of individual survival. They are vulnerable to fire.

t-Virus leeches
The source of these leeches is unknown, although they are likely associated with the Giant Leech that inhabited the sewers beneath Raccoon General Hospital and they may emerge from the egg clusters it laid there. Compared to the Progenitor leeches, these look more like garden-variety leeches, and a hospital employee associated with Umbrella postulates that they are unrelated to Marcus's research. These leeches are unable to form mimicry bodies, but if they encounter a sufficiently fresh corpse, they can colonize it to create a Leech Zombie. Like all leech specimens, they are vulnerable to heat.