Pia is a 13-year-old German immigrant living in a cramped Philadelphia apartment. She possesses a unique, secret sensitivity that allows her to feel the sickness or pain of others through physical contact. This ability isolates her socially and makes her actively shrink from human touch. Following the sudden loss of her mother to the influenza epidemic, she assumes total responsibility for keeping her infant twin brothers alive.

Bernice is a 20-year-old war widow living across the street from the Lange family. Consumed by the loss of her infant son to the flu, she seeks someone to blame, focusing her intense anger on immigrants and people of color. She justifies her increasingly deceitful actions under the guise of American patriotism and religious duty, exploiting the chaotic conditions of the epidemic for her own purposes.

Finn is an Irish immigrant boy with thick copper hair who lives down the hall from Bernice. As one of the few people who accept Pia despite the widespread anti-German sentiment, he provides her with much-needed companionship. He faces his own severe hardships when the influenza virus strikes his entire family, forcing him onto the streets.

Mrs. Hudson is the kind-hearted matriarch of a household that requires domestic help to manage its many children. She provides a generous and welcoming environment for those in her employ, differing greatly from the harsh conditions of the city's poorer districts. She follows her husband's strict safety protocols to protect her family from the raging epidemic outside their doors.

Dr. Hudson is a physician and war veteran who lost an arm in combat. He possesses a deep understanding of the influenza virus and implements rigorous precautions to shield his family. He maintains a rational, open-minded approach to medicine and shows a willingness to listen to unorthodox observations about patient health.

Mrs. Lange is a German immigrant mother who works hard to prove her family's loyalty to America during World War I. She insists her children speak English in public to avoid anti-immigrant hostility. She is fiercely protective of Pia, actively trying to shield her daughter from uncomfortable physical contact in crowded spaces.

Ollie and Max are four-month-old infant twins with bright cobalt-blue eyes. They are entirely dependent on their family for survival. Their vulnerability makes them central to Pia's urgent efforts to keep her family together as the flu ravages their impoverished neighborhood.

Rebecca is a young woman seeking domestic work who arrives unexpectedly at the Hudson household. She carries a profound sadness and demonstrates an intense, emotional interest in the young children she encounters while working there.

Mr. Lange is a German immigrant who moved his family from the countryside to Philadelphia for better opportunities. Despite facing local prejudice, he enlisted in the American military to fight in World War I, leaving his wife to manage the household in his absence.

Mother Joe is the stern head of the orphanage where many children are placed during the epidemic. She uses strict discipline and conditional promises to maintain order among the desperate, grieving children under her control.

Sister Ernestine is a harsh, abusive nun working at the orphanage. She uses threats, physical punishment, and intense intimidation to control the children, showing zero compassion for their traumatic circumstances.

Mary Helen is a schoolyard bully who targets classmates who seem different. She frequently mocks those who isolate themselves, using physical intimidation until the influenza outbreak disrupts the community.

The Pattersons are an elderly married couple who offer their time to watch young children in their building. They are trusting neighbors who extend kindness to individuals they believe are struggling war widows.

Leo is the two-month-old son of Dr. and Mrs. Hudson. He is fragile and requires careful attention, prompting his parents to hire extra help to ensure his well-being.

Elizabeth is the 22-month-old daughter of Dr. and Mrs. Hudson. She requires careful attention and childcare as her family isolates themselves from the spreading epidemic outside their doors.

Sophie is the three-year-old daughter in the Hudson family. She benefits from her family's strict quarantine measures and the comfortable environment her parents maintain during the crisis.

Margaret is four years old and the eldest daughter of Dr. and Mrs. Hudson. She actively participates in the household dynamics and observes the arrival of new caretakers in their home.

Cooper is a young infant brought to the Hudson household in need of a home. His arrival brings a mix of hope and tension into the grieving family's lives.

Wallis is the infant son of Bernice Groves who succumbs to the influenza virus. His tragic death catalyzes his mother's profound grief and sparks her ensuing destructive actions.

Miss O'Malley is a stern woman responsible for transporting orphaned or displaced children to the local asylum. She uses physical force to ensure compliance from the children in her custody.

Edith is a young orphan girl assigned to the infant ward at St. Vincent's. She provides Pia with insights about the staff and visiting nurses, understanding the unspoken prejudices that govern the orphanage.