Despite efforts of international and national health authorities, immunization coverage and timeliness of vaccination against dangerous childhood diseases have been adversely affected by parental hesitation to vaccinate their children in high-income countries. Literature shows that social and political processes and shifts in conceptual structures, such as emerging views linked to health and 'natural' lifestyles, have shaped parents' immunization decisions. This paper investigates how Swiss parents argued along the lines of a natural development of the child to explain their critical attitudes towards immunization against measles and other childhood diseases.; A total of 32 semi-structured interviews were conducted with parents of children between 0 and 16 years of age who decided not to fully immunize their children. The interviews were analyzed using qualitative content analysis and an interpretative approach.; Parents built their arguments against immunization on a strong faith in the strength of the naturally acquired immune system. Childhood diseases were not perceived as a threat but as part of the natural way to reinforce the body and to acquire a "natural" and thus strong immunity. Parents understood immunization as an artificial intrusion into the natural development of the immune system and feared overloading the still immature immune system of their young children and infants through current vaccination schemes.; In the context of emerging trends towards natural lifestyles and ideas of holistic health in Switzerland and Europe, where many well-informed parents express concerns towards vaccinating their children, public vaccination strategies require reconsideration. Public immunization schedules need to acknowledge parents' wish for more flexibility and demand for an individualized patient-centered approach to immunization.