Final answer:
To determine a causal relationship between an etiologic factor and a disease, exposure to the factor must come before the disease onset, and a higher prevalence or incidence in exposed groups must be observed, though the factor doesn't need to be found in all disease cases.
Step-by-step explanation:
For an etiologic factor to be determined to be causally related to a disease, several criteria must be met. One key requirement is that exposure to the etiologic factor must precede the onset of the disease, establishing a temporal relationship. Additionally, persons exposed to the etiologic factor typically have a higher prevalence or incidence of disease compared to those not exposed. This suggests that the factor is linked to an increased risk of the disease. However, it's not necessary for the etiologic factor to be found among all cases of the disease, as other causes and contributing factors may be involved.
The process of establishing causation usually involves comparing the prevalence and incidence of disease in exposed versus unexposed groups, utilizing methods such as case-control studies to determine relative risk, and ensuring that the suspected pathogen is consistently associated with the disease and can reproduce the disease symptoms when a healthy subject is infected.