Etiology is study of the cause of disease. More specifically, etiology is the sum of knowledge regarding a disease, or knowledge of all pathological processes leading to a disease. There may be just one causative agent, but no one cause of disease. For example, as stated in Boyd's Introduction To The Study Of Disease, we know tuberculosis is caused by the tubercle bacillus. Many people are exposed to the tubercle bacillus yet only one may develop the disease. The bacilli can also lay dormant in the body for many years and became active as the result of an intercurrent infection (occurring during the course of an existing disease), prolonged stress, or starvation. While some diseases have no known cause, there are several factors that may contribute to, or predispose someone to a disease. Many variables must be considered when looking at the cause of a disease such as, gender, age, environment, lifestyle, heredity, allergy, immunity, and exposure.