[1] The word etiology is derived from the Greek αἰτιολογία, aitiologia, "giving a reason for" (αἰτία, aitia, "cause"; and -λογία, -logia).
[4] The Ancient Roman scholar Marcus Terentius Varro put forward early ideas about microorganisms in a 1st-century BC book titled On Agriculture.
The infectious agents that cause disease fall into five groups: viruses, bacteria, fungi, protozoa, and helminths (worms).
An example of all the above, which was recognized late, is that peptic ulcer disease may be induced by stress, requires the presence of acid secretion in the stomach, and has primary etiology in Helicobacter pylori infection.
Many chronic diseases of unknown cause may be studied in this framework to explain multiple epidemiological associations or risk factors which may or may not be causally related, and to seek the actual etiology.
[citation needed] Conversely, a single etiology, such as Epstein–Barr virus, may in different circumstances produce different diseases such as mononucleosis, nasopharyngeal carcinoma, or Burkitt's lymphoma.
This is distinct from a phenotype, which is any observable characteristic or trait of a disease, such as morphology, development, biochemical or physiological properties, or behavior, without any implication of a mechanism.