The search for the causes (etiologies) of human diseases goes back to antiquity.Hippocrates, a Greek physician of the 4th and 5th centuriesbce, is credited with being the first to adopt the concept thatdisease is not a visitation of the gods but rather is caused by earthly influences. Scientists have since continually searched for the causes of disease and, indeed, have discovered the causes of many.

In the development of a disease (pathogenesis) more is involved than merely exposure to a causative agent. A room full of people may be exposed to a sufferer from acommon cold, but only one or two may later develop a cold. Many host factors determine whether the agent will induce disease or not. Thus, in the pathogenesis of disease, the resistance, immunity, age, and nutritional state of the person exposed, as well as virulence or toxicity of the agent and the level of exposure, all play a role in determining whether disease develops.

In the following sections the many types of human disease will be divided into categories, and in each only a few examples will be given to establish the nature of the process. These categories are divided on the basis of the presumedetiology of the disease. Many diseases are still of unknown (idiopathic) origin. With others the cause may be suspected but not yet definitively proved. In a few instances the discovery of the etiology of a disease represents the individual achievement of a solitary investigator who may have worked many years on the problem; the story ofLouis Pasteur and the discovery of the cause ofanthrax is a classic example. More often the individual investigator who makes the final breakthrough stands on the shoulders of hundreds of earlier workers who provided bits and pieces of knowledgevital to the final understanding.