The present study examined children’s and adults’ explanations about the relations between various factors (contagion, imbalanced diet, lack of sleep, mental depression, cold weather, and immanent justice) and the onset of illness. Six-, 9-, 11-year-old children and adults very frequently judged that contagion and cold weather would make an individual contract a cold. They also tended to judge that lifestyle habits such as nutrition and sleep are causal factors. When asked why these factors led to illness, children and adults frequently referred to “germs” and “resistance” factors. In resistance explanations, given factors were assumed to cause illness by weakening an individual’s resistant power, which was expressed as vital force, power, energy, physical strength, immune system, or stress. Coexistence of “germ” and “resistance” explanations were more evident among adults than children, and among adults with a higher education level.