Infectious diseases are disorders caused by organisms — such as bacteria, viruses, fungi or parasites. Many organisms live in and on our bodies. They're normally harmless or even helpful. But under certain conditions, some organisms may cause disease.

What is the origin of illness?

Illness derives from the Middle English adjective ill, which in turn is from the Old Norse term illr (bad, not related to evil) whose origin is, in turn, lost in the mists of time. Now, consider how illness has been used over the centuries in medical practice.

What is the origin and cause of illness?

The etiology of a disease is its cause or origin. Etiology is also the name for the study of the causes of diseases. It can also refer to the study of the cause of things in other fields, such as philosophy and physics. But it is most commonly used in the context of medicine.