Aims and Objectives: To explore the onset of sepsis based on patients' and family members' experiences.

Background: Knowledge about the onset of sepsis is limited among patients and their families, which makes early recognition of sepsis difficult. Previous studies argue that their stories are important to recognising sepsis and reduced suffering and mortality.

Design: A descriptive design with a qualitative approach was used.Methods: In total, 29 patients and family members participated in 24 interviews with open-ended questions, including five dyadic and 19 individual interviews. The interviews were conducted during 2021, and participants were recruited from a sepsis group on social media. A thematic analysis based on descriptive phenomenology was performed. The study followed the COREQ checklist.

Findings: Two themes emerged from the experiences: (1) When health changes into something unknown, including the two subthemes; Bodily symptoms and signs being vague but still tangible and Feelings of uncertainty, and (2) Turning points when warnings signs are deemed as serious, including the two subthemes Passing borders when feeling out of control and Difficulties understanding the seriousness.

Conclusions: Patients' and family members' stories of the onset of sepsis indicate that symptoms and signs appeared insidiously and then noticeably worsen. The symptoms and signs seemed not be attributed to sepsis; instead, there was uncertainty about what the symptoms and signs meant. It was mainly family members who possibly understood the seriousness of the disease.

Implications for the profession and patient care: Patients' experiences of their symptoms and signs and family members' unique knowledge of the patient, indicate that healthcare professionals should listen and try to understand what the patient and family members are telling and take their concerns seriously. How the condition appears, and family members' concerns are important pieces of the assessment to recognise patients with sepsis.

Patient or public contribution: Patients and family members contributed to the data collected.