The primary assumption in assigning a role to psychosocial factors in disease susceptibility is that, given a reasonably large sample of persons all of whom are equally exposed to a particular pathogen or disease-causing vector, some will become sick sooner. Psychosocial factors may be defined as those aspects of the individual's life situation which have a high content of intrapersonal and interpersonal significance, are closely entailed in meeting basic roles in family and community, and carry broad markers of "success" or "failure," thus being tied to morale or self-esteem. The hard research to discover psychosocial precursors as necessary, if not sufficient, causal factors in physical illness is new. The ancients spoke of "Mens sana in corpore sano," a healthy mind in a healthy body. The accumulated research to date on psychosocial factors in disease susceptibility has served to validate the crude formulations.