ABSTRACT: Post-infectious fatigue syndrome (PIFS), also known as post-viral fatigue syndrome, is a complex condition resulting in physical, cognitive, emotional, neurological, vocational and/or role performance disabilities in varying degrees that changes over time. The needs for health care resources are high, and costly, as is the economic burden on the affected individuals. Many factors may impact the trajectory, and frequently PIFS develops into a chronic condition. Health professionals lack understanding and knowledge, which results in delayed diagnosis, lack of recognition, appropriate treatment, support and practical help. The aim of our study was to explore, from the perspective of persons who had lived with PIFS for four years following an outbreak of Giardia l. induced enteritis, factors that may have impacted their illness trajectory and how these factors had played a role during different phases.In this retrospective exploratory qualitative study a group of 26 affected adults between 26 and 59 years old were selected for in-depth interviews. A maximum variation sample was recruited from a physician-diagnosed cohort of persons with PIFS enrolled at a tertiary outpatient fatigue clinic. The interviews were audio-recorded, transcribed verbatim and subjected to qualitative content analysis.Unhelpful and helpful factors were associated with the healthcare system, health professionals and the affected persons were experienced as having an impact on the trajectory. External impacting factors which are related to the health care system, providers and the social security system are misdiagnosis, trivialization of symptoms, unhelpful advice, delayed diagnosis and lack of appropriate help. Internal impacting factors related to the affected individuals were lack of knowledge, overestimating functional capacity, assuming the condition will pass, ignoring body signals and denial. A model of impacting factors in each phase of the trajectory is presented.Unmet needs may result in unnecessary disability and high societal and personal costs. Enhanced knowledge of impacting factors in each phase of the trajectory may contribute to more timely and tailored health care services and less use of health services. Increased functional capacity, improved health and ability to work or study may reduce the societal costs and the economic burden for the affected individuals.