While the decision to immunise young children can be understood to some extent in the context of perceptions of severity and susceptibility to disease and benefits and barriers to immunisations, the theories of risk perception and decision-making add a depth of understanding to the differences found between these parents in terms of their perceptions and interpretations of what is risky and what is not. Two aspects in particular appear to be important: the familiarity or unfamiliarity with the disease and perceived control over risks or outcomes. Thus, perceptions of severity of disease are influenced by the unfamiliarity of the disease and/or the perception that these diseases will have uncontrollable outcomes. That is, diseases with which parents are least familiar are perceived as more severe than those with which parents are more familiar and therefore worth taking preventive action. Being a familiar disease contributed to delays in immunisation, or not immunising as these familiar diseases were not considered to be severe.