In a prospective study of a disease, the cohort originally selected consists of:

In a prospective study, researchers typically follow a group of individuals over time to observe the development of a disease or health outcome. The cohort originally selected consists of individuals who do not have the disease being studied at the outset. This is crucial because the primary aim of a prospective study is to identify risk factors or exposures that may contribute to the later onset of the disease.

By starting with persons who are disease-free, researchers can observe how the presence or absence of certain factors influences the incidence of the disease over time. This design allows for a clear temporal relationship between exposure and outcome, providing valuable insights into causative factors.

In contrast, selecting participants who already have the disease or who have the factor of interest would complicate the study's design and interpretation, as it would no longer effectively isolate the impact of those factors on the development of the disease.