Etiology is the description of how a particular disease causes the symptoms and progression of that disease. Knowing this information is important in developing a treatment for that disease.
Although many disease mechanisms are well understood (down to the molecular level), there are still diseases with no well established mechanisms that are still serious or even life threatening. There are also diseases where the etiology is known for the most part, but the exact cause or mechanism is still unknown.
For example, the etiology of the common cold is well understood, from infection to why it will resolve itself - The disease is caused by a virus which persists for several days outside the human body. When the virus reaches the upper respiratory tract, it begins to penetrate the tissues of the respiratory tract, destroying cells and making large numbers of copies of itself. However, this sets off a histamine response, which results in the upper respiratory tract filling with mucous. This raises the temperature in the respiratory tract, making it more difficult for the virus to survive. The body produces antibodies to the virus within about seven days, which soon overwhelm the virus and the disease runs its course.
However, diabetes mellitus has a well understood etiology except for two matters - why the islets of Langerhans stop producing insulin and why cells start to resist the action of insulin. Although risk factors are well understood for diabetes, they do not explain why some patients do not develop the disease, or why some patients with no risk factors do develop the disease.