Marlena Diamond is killed by a parasite's bite in the film. The bite in the subway caused Marlena to bleed from the eyes and vomit blood from the mouth. Before Marlena explodes, a soilder is wheeled by the group, chest having gaping hole, and the medical officers saying "another bite", foreshadowing Marlena's fate.
The parasite is comprised of a crustacean-like carapace, several pairs of claws, and arms. A deep blue-purple muscular membrane stretches between the top and lower jaws. When a human is bitten, the victim becomes ill and bleeds profusely, and shortly after this the torso expands and explodes.
The Parasite's jaw is unique in that the top half of the head is the mandible, moving up from the lower jaw to open, as opposed to the reverse with almost all terrestrial animals. The top and lower jaws end in serrated edges and also have four pairs of eyes each.
The parasites' movement is unique in that they have the ability to climb walls and on ceilings. Their leg movement is similar to that of spiders, and have the ability to jump great distances, similer to that of fleas which they are apperantly based on acording to the concept art.
BiteSaliva in the parasites bite causes the victim to feel disoriented, bleed from the eyes and finally cause the victim to inflate, and have their stomach explode. Pouring water on the wound speeds up the violent explosion, as shown by Marlena, who exploded 20 minutes after the bite. Most victims died in 20-40 minutes.
- It is not uncommon for hematophagic parasites currently known to science to inject their hosts with mild anesthetic anticoagulants that numb the pain and prevent the host's blood from clotting (mosquitoes do this, and our allergic reaction to that saliva causes the bumps). Given the size and resilience of the monster, it is not surprising to see blood pressure of a human injected with the anticoagulant to rise to such astronomic levels that it simply rips their body from inside out.
- Another theory is that the poison causes extreme buildup of fluid or gases until the body can no longer take it.
- Some think it is a form of the Ebola Virus. They believe it was brought into Africa thousands of years ago possibly by the monster's parents. This would explain why they were wearing chemical suits to protect from the spread of this virus, but Ebola is merely a hemorrhagic virus that causes severe bleeding, and only in a few cases. It doesn't cause anyone's stomach to explode violently.
- Another more complicated theory is the venom the parasites produce is well tuned to the depths of the ocean that when Marlena was bitten the venom did not have the pressure to keep it at its normal effects so it expanded and caused her stomach to explode.