At first glance, Thrax appears to be collected, communicative, soft-spoken, suave and falsely polite, but it only serves to highlight the loathsome monster he really is and enhance the threat he poses. To his core, he is very arrogant, boastful, brutal, violent, ambitious, completely remorseless and gleefully sadistic.
Thrax is notable for being a very dark and threatening villain in an otherwise lighthearted comedy. As a serial killer of the most heinous kind, he lives for the thrill of murder, which he regards as a challenge. He greatly enjoys causing death and suffering to people and cells alike, and he is also intensely proud of his killing-spree, often bragging about his many victims, among other displays of macabre dark humor. He always keeps a chain made of the DNA of all his victims as a creepy keepsake, and often singsongs Peggy Lee's "Fever" song when spreading his disease as a twisted sense of irony.
Thrax craves to attain infamy in medical books by breaking all records of deadliness. Whenever he enters the body of a new victim, he fixes himself a deadline to kill his target, each shorter than the previous. He has a volatile temper despite his often suave ways and often flies into short yet fiery rages at the prospect of obstacles. He prides himself in having never been spotted by his victims' organism before (until he entered Frank and Osmosis found out about him), and opposition only makes him more reckless and unhinged.
Despite his violent tendencies, Thrax has shown to be knowledgeable about human anatomy; knowing how to blend among common viruses, how to cover his tracks, and where to strike as efficiently as possible. Yet, his obsession about keeping his perfect record at all costs would lead to his downfall.
In the beginning of the story, Thrax enters the body of Frank DeTorre, a widower without the slightest regard for hygiene and health, when the latter eats an egg that was just spit by a monkey into a muddy floor. By doing so, he infiltrates Frank City, a huge metropolis inside Frank's body in which each of his body cells are living a life identical to that of humans, with white blood cells as cops and germs as criminals.
Thrax casually murders the two witnesses of his arrival, and goes to the germs headquarters in Frank's armpit. There, he quickly takes control of the germs gangs by killing their leader; and plans to take advantage of Frank's poor health condition to operate undetected.
The titular primary protagonist is Osmosis "Ozzy" Jones, one of Frank's white blood cells who was demoted from the police after he activated the emergency evacuation (i.e. throwing up), to get rid of germs from polluted oysters, which was seen as "unnecessary force".
He gets reluctantly teamed up with a rule abiding cold pill named Drix (Drixenol) to investigate on a throat inflammation (in fact Thrax's doing), and they discover that something far more dangerous is going on. However, the incompetent and election-obsessed Mayor Phlegmming, who is to blame for Frank's disastrous life-style, dismisses their warnings and states that this is nothing more than a "common cold".
Jones uses his body-altering abilities to pass as a germ and infiltrates a meeting of Thrax and his thugs inside a zit, to get in on what is going on. There, he discovers about Thrax's plan to set a record with Frank's death. When Ozzy is caught, Drix barges him just as Thrax was about to incinerate him. After a brief battle, the zit is destroyed in the explosion of a medicine grenade, killing most of Thrax's gang and seemingly Thrax himself as well.
Yet, the virus has survived and is hiding in a toenail with his remaining thugs. As Thrax prepares to get back on schedule, one of his gangsters suggest they incubate for a while as they are few in numbers (which would mean he won't be able to break his record). Angered and disillusioned, Thrax kills his remaining mobsters in an explosion, heading off to finish things as a lone wolf.
Phlegmming fires both Ozzy and Drix, but they keep investigating nonetheless, until they can be sure that Thrax is really gone. They go to the memory zone of the brain and discover that Thrax has sneaked in. Meanwhile, Leah Estrogen, the mayor's assistant and Ozzy's love interest, starts suspecting that something is amiss, but even when he can no longer ignore the situation, Phlegmming keeps underestimating it. Leah starts her own investigation in the brain and stumbles upon Thrax, who stole a DNA bead from Frank's hypothalamus, causing his body temperature to skyrocket, and the entire city to be devastated in a fire, under Phlegmming's disbelieving and remorseful eyes.
Despite her activating the alarm and putting up quite a fight, Thrax takes her as a hostage to force Ozzy and Drix, who caught up with them, to let him go, but not before Drix could shoot Thrax’s claw with a freezing blast, preventing him from using it.
As Frank is being rushed into the hospital in critical condition, Thrax triggers a sneeze to exit his body, followed by Ozzy who used Drix's cannon to propel himself after his foe, landing in Frank's daughter Shane's eye.
Ozzy and Thrax engage a fistfight, with Ozzy altering his body structure to better fight his foe, fighting him evenly. Thrax manages to catch Ozzy and strangles him with the DNA chain, gloating that he will break his record with Shane as soon as Frank dies, but Ozzie traps him on the girl's fake eyelash that is about to fall.
Ozzy clings to Shane's real eyelash, with the DNA chain still around his neck, while Thrax falls into a bottle of rubbing alcohol and dissolves to death. Ozzy then uses Shane's teardrop to re-enter Frank's body and replace the stolen DNA, saving Frank's live in extremis.
As Frank is saved and vows to adopt a healthier life-style, Phlegmming is impeached and Ozzy is appointed as an elite cop, with Drix as his partner and Leah as his girlfriend.