Julia is a 29-year-old nurse working in a major Dublin hospital during the 1918 influenza pandemic. Specializing in midwifery, she oversees the heavily understaffed Maternity/Fever ward, an ad-hoc room created to separate infected pregnant women from the general population. She is highly practical, methodical, and dedicated to hygiene, prioritizing her patients' physical comfort and safety above hospital politics. Outside of work, she lives in a small house with her war veteran brother.

Bridie is a 22-year-old volunteer helper at the hospital. She lives at a local convent with other young women, working to pay off the cost of her upbringing in the orphanage system. Though she lacks formal medical education and initially appears ignorant of basic hospital procedures, she is brave, deeply empathetic, and a fast learner. She possesses a natural bedside manner that comforts the frightened pregnant patients.

Dr. Lynn is an experienced physician helping at the understaffed hospital. As a female doctor, she is a rarity in 1918 Dublin. She is an outspoken activist who advocates for Irish independence and better living conditions for the poor, beliefs that draw police attention. She approaches her medical duties with extreme competence, willingly trusting the nurses' judgment and overriding rigid hospital hierarchies.

Tim is Julia's 26-year-old younger brother. He is a veteran of World War I who receives a government pension for his service. The psychological trauma of the trenches left him entirely mute, though he remains physically unharmed. He spends his days tending the household garden, cooking meals for his sister, and caring for a pet magpie.

Groyne is an orderly at the hospital responsible for moving patients and transporting the deceased to the morgue. Like many of the male staff, he is a World War I veteran. He frequently sings music-hall numbers and makes crude, inappropriate jokes while working, a habit that initially repulses his coworkers. Under his boisterous exterior, he carries intense personal grief.

Sister Luke is a Catholic nun assigned to oversee the Maternity/Fever ward during the night shifts due to a severe shortage of trained midwives. She resides at a local convent that also operates as a home for orphans and boarders. She carries strong prejudices regarding unwed mothers and relies heavily on religious moralism when interacting with patients.

Ita is a 33-year-old pregnant woman infected with the influenza virus. She is experiencing her twelfth pregnancy, though she only has seven living children. The virus causes her to become highly delirious, uncoordinated, and reliant on the nurses for basic bodily functions.

Delia is a 20-year-old expectant mother staying in the quarantine ward. Coming from a Protestant background and possessing more financial security than the typical ward patient, she is restless and frustrated by her confinement. She frequently demands to be sent home, failing to grasp the severity of her mild flu symptoms.

Mary is a 17-year-old pregnant woman experiencing early labor alongside an influenza infection. She is highly inexperienced regarding childbirth, holding naive beliefs about human anatomy. Outside the hospital, she is the primary caretaker for her five younger siblings and lives in a difficult, abusive marriage.

Honor is a young expectant mother transferred to the hospital from a local home for unwed mothers. She is pregnant out of wedlock for the second time, a fact that draws harsh judgment from the religious staff. She is deeply pious, constantly praying, and refuses pain medication as a form of self-imposed penance.

Gladys is a nurse at the hospital who specializes in eye and ear care. She is a longtime friend of Julia's, having trained alongside her when they were both in their early twenties.

Dr. Prendergast is a physician on duty at the hospital. He issues basic instructions for symptom management, expecting the nursing staff to follow his orders without contradiction, maintaining the strict traditional hierarchy of the medical field.

Dr. MacAuliffe is a young general surgeon filling in at the hospital. He lacks practical experience with pregnant women and childbirth, leading him to quickly recommend invasive and potentially damaging surgical interventions when labor fails to progress smoothly.

Father Xavier is a Catholic priest who visits the hospital wards. Due to the high death toll and staffing shortages during the pandemic, he ministers to all patients regardless of their specific religious denominations.

Nichols is an orderly working at the hospital. Like many orderlies on staff, he is a veteran of World War I. He wears a metal mask to conceal a severe facial injury he sustained during combat.

O’Shea is an orderly at the hospital. He is a war veteran who suffers from a persistent physical tremor, an injury resulting from his time on the front lines.

Miss Ffrench-Mullen is Dr. Lynn's close companion and partner. She shares a home with the doctor, and the two women hold ambitious plans to eventually open a specialized hospital and country home dedicated to treating the infants of the poor.