Disease is an abnormality in the structure or function of the body. The definition of ‘abnormal’ is difficult. On the one hand, all animals show extensive variation in size, shape and physiology. What is normal? On the other hand, some diseases are so common that almost all individuals show some sign of them. Do these still qualify as diseases? Allowance has therefore to be made for a normal range of variation that does not lead to impairment of function but, once a structure or process cannot operate efficiently, it is considered to be in a state of disease. The process through which a disease is caused is known as its aetiology, and the site at which an abnormality occurs is known as a lesion. This may only be a chemical change. It may be confined to soft tissue, or it may involve widespread hard tissue destruction and repair. Usually, the bone and dental tissues that are preserved in archaeology represent only one part of the lesion. The rest has to be reconstructed and this makes diagnosis difficult. This chapter includes dental caries, periodontal disease, periapical inflammation and injuries. It does not include the defects of enamel hypoplasia even though, if they are pronounced, they may predispose to caries or tooth fracture, because they are dealt with in Chapter 2.

Disease is part of ecology. It represents the impact of the environment and the body's reaction to it. This makes disease a very useful source of information in archaeology.