A disease is a condition where the body does not function correctly.
Diseases are caused by the body being overwhelmed by toxic substances beyond its ability to eliminate them in a normal way. Toxic materials may accumulate from unexpelled body wastes that are generated as a part of metabolism or from external materials that are ingested and retained due to the body's inability to eliminate them. Disease also refers to bacterial and viral infections of the body.
The etymology of the word comes from the English dis-ease.
In the scientific arena, the term disease can be applied to any pathological condition presenting a set of signs and symptoms. In the scientific sense, a disease is not necessarily harmful.