The
parasite featured in the novel is portrayed as being the source of many
myths and
legends. The symptoms in a carrier are only a craving for fresh meat and sexual arousal around the clock, but the symptoms of the normally affected are quite different. The parasite causes
dementia and
cannibalism, which were suggested as causing legends of most notably
vampires, but also
werewolves and
zombies, among others. The
protagonist describes the
superhuman strength, speed, and vision as the human body simply operating on emergency power. He describes it as being the same as a mother protecting her child, specifically the rush of
adrenaline provided so that she can act as she needs to. The parasite simply causes the body to act on this emergency adrenaline all the time, requiring extra food. The superior abilities are shown in the novel several times by Cal. He claims to need to tape over the light of his
DVD player to be able to sleep at night and can read fine print by starlight. He jumps fearlessly from a balcony to the one beside it, which causes suspicion from Lace, not afflicted with the parasite at the time. The parasite can be spread either directly from human to human, or via an infected rat or cat. Rats are shown to be attracted to infected persons. The parasite also seems to give those infected with it either immortality or at least extreme longevity, as shown by the Nightmayor, who had lived for centuries. Two different strains were featured in the novel. The first to appear is a new strain, the symptoms and possible treatments for which have been known to Cal prior to the events of the novel. The body can react in two ways to this strain. The more common one causes dementia, cannibalism and a painful response to light of any kind. Other symptoms involved hatred of what was once loved, which can range from a singer to a skyline, which can be used to capture the infected, and nearly-total reliance of primitive instincts. Seemingly-supernatural strength is present in this response, but it is used most often in instinctive reactions and basic needs, primarily food. To keep the parasite spreading through the population of the earth, the infected can spread the parasite to rats, who defend them and continue to spread the parasite even if their progenitors cannot. The out-of-control cannibals who contract the disease and exhibit these symptoms are hunted down by the Night Watch, a group which fights to contain this strain of the parasite. There is no true cure for this reaction to the strain, and hospitalization coupled with an experimental plan of drugs is the only known treatment to contain a threat, apart from imprisonment. The second possible reaction to this strain which is the much less common, involves a more subtle response. People who contract this strain of the parasite and receive this reaction are known as "carriers" of the disease since they do not suffer from its worst effects but have the possibility of spreading the disease nonetheless. A craving for meat remains, involving that of the human variety, but a carrier of the strain is no longer forced to succumb totally to their instincts. It is possible for the carriers of this strain to live a near normal life, because the stronger symptoms are absent. However, they must shy away from any close contact with others humans, involving simple things such as kissing and sharing a toothbrush, for fear of spreading the disease further. The worst of the remaining reactions, primarily sexual arousal and cravings for meat, can be overcome. The carriers are compared to
Mary Mallon, most commonly referred to as
Typhoid Mary, by the protagonist. The second and older strain presents itself in a different manner. To the untrained eye, it may appear to be the same as the new strain. However, this strain is much more manageable. It appeared in ancient times, long before the new strain existed, and aided in defeating the attack of a race of large worms. By infecting as many people as possible, an army was created to defeat the giant worms threatening humanity. This older strain all but disappeared after it was no longer needed, existing only in a brood of rats living underground. When the giant worms that once threatened the existence of the human race return, they force the infected rats to the surface who then spread the old strain back into the human population. The strain is used to defeat the giant worms once again, allowing the human race to survive, but not without a certain degree of damage done to the world. The dementia and cannibalism are present towards the beginning of the infection, and is much more easily treatable. Old methods which were used in the last large-scale parasite infection are rediscovered and used, such as
garlic and
rosemary, which is evidently meant to represent the beginnings of vampire legends involving garlic as protection. The parasite is further explained in
The Last Days, which adds another dimension to the parasite. It is revealed that in ancient times, there were certain "singers" that could call up giant worms from within the Earth so that they could be more easily killed. That allowed the army of fighters against the giant worms to attract the worms under their preferred conditions, such as time and location, with sufficient preparation. The singers were able to attract the worms by singing words that were guided by the parasite living within them, sounding meaningless to the average human, but captivating nonetheless. Dr. Prolix ("The Shrink"), an age old carrier of the parasite has vague memories of seeing one of these singers calling a giant worm up to be killed, and
Minerva, a prominent character in
The Last Days, has inherited that ability from the parasite positives of centuries ago. She experiences receiving these songs from underground in her home, where old burial grounds are located, giving a possible origin to that ability. ==Other parasites==