Etiology

Etiology is the study of the causes or origins of diseases and other phenomena. In medical contexts, it refers to the factor or set of factors responsible for the initiation of a disease or condition. Etiology encompasses biological agents, genetic predispositions, environmental exposures, and behavioral and social factors that contribute to disease development.

Etiology is distinct from pathogenesis, which describes the biological mechanisms by which a cause leads to disease. A condition may have one principal etiology or multiple etiologies that interact over time. Some diseases have well-defined etiologies, others are multifactorial or of uncertain origin.

Etiologies are commonly categorized as infectious (caused by pathogens such as viruses, bacteria, fungi, or parasites); genetic or developmental (inherited mutations, chromosomal abnormalities, congenital defects); metabolic or toxic (defects in metabolism, exposure to toxins); immune-mediated and inflammatory; and environmental or lifestyle factors (smoking, diet, radiation). In many chronic diseases, etiologies are multifactorial, with several agents contributing to risk.

Determining etiology relies on epidemiology, clinical observation, and experimental evidence. Historically, Koch's postulates guided infectious etiologies; modern causal inference uses criteria such as temporality, strength of association, consistency, and biological plausibility to assess etiologic relationships (Bradford Hill criteria).

Examples illustrate the range of etiologies: Helicobacter pylori is the etiologic agent of many peptic ulcers; mutations in BRCA1 and BRCA2 increase heritable cancer risk; tobacco smoke is a major etiologic factor for lung cancer.