BACKGROUND: It is unknown whether there is a general factor that accounts for the propensity for both physical and mental conditions in different age groups and how it is associated with lifestyle and well-being. METHODS: We analyzed health conditions data from the Millennium Cohort Study (MCS) (age = 17
N = 19,239), the National Child Development Study (NCDS) (age = 44
N = 9293), and the English Longitudinal Study of Ageing (ELSA) (age ≥ 50
N = 7585). The fit of three Confirmatory Factor models was used to select the optimal solution by Comparative Fit Index, Tucker-Lewis Index, and Root Mean Square Error of Approximation. The relationship among RESULTS: Supporting the existence of the CONCLUSIONS: Contrary to the traditional dichotomy between mental and physical conditions, our study showed a general factor underlying the comorbidity across mental and physical diseases, related to lifestyle and well-being. Our results inform the conceptualization of mental and physical illness as well as future research assessing risk and pathways of disease transmission, intervention, and prevention. Our results also provide a strong rationale for a systematic screening for mental disorders in individuals with physical conditions and vice versa, and for integrated services addressing multimorbidity.