Mitty is a wealthy high school student living in a secure Manhattan apartment building. He usually coasts through his classes with minimal effort, preferring to socialize, listen to music, and watch sports. His complacent attitude shifts when he finds old smallpox scabs in a medical book, sparking a sudden and intense interest in his biology project. He loves his city and enjoys chatting with the people in his neighborhood.

Olivia is a studious and dedicated high school student who consistently stays ahead of her coursework. She feels more comfortable around books than people, spending her free time reading and studying. She provides Mitty with resources and guidance for his assignment, often marking specific pages to make his work easier.

Derek is one of Mitty's closest friends at their private Manhattan high school. He is a dedicated student who pays close attention to current events, particularly news regarding national security and bioterrorism. He enjoys Mitty's company and humor, frequently teasing him about his lack of academic effort.

Mrs. Blake is an interior decorator who lives a comfortable life in New York City with her family. She is highly attentive to her son's daily schedule, actively monitoring his attendance to ensure he gets to his classes. She purchases old books for her design projects, inadvertently providing Mitty with the material for his biology assignment.

Mr. Blake is a wealthy New York resident who provides his family with a comfortable life, including a private school education for his son. He enjoys relaxing on the weekends, particularly watching UConn basketball games on television. He holds high expectations for his son's future but remains largely unaware of Mitty's academic struggles.

Mr. Lynch is a biology teacher at Mitty's private high school. He maintains strict academic standards, requiring his students to use physical books rather than relying solely on the internet. He actively encourages his students to expand their understanding of science by interviewing experts and avoiding plagiarism.

Dr. Graham is an epidemiologist working for the Centers for Disease Control and Prevention (CDC). He operates alongside federal agents to investigate potential public health threats. He provides scientific expertise regarding the viability of old viruses and the mathematical likelihood of infection.

Dr. Larkin is the headmaster of the private Manhattan high school attended by Mitty, Olivia, and Derek. He manages the administrative side of the school and coordinates with outside authorities when situations require external intervention.