NATURAL HISTORY

The natural history of disease refers to the progression and projected outcome of a disease without medical inter­vention. By studying the patterns of a disease over time in populations, epidemiologists can better understand its natural history. A knowledge of the natural history can be used to determine disease outcome, establish priorities for health care services, determine the effects of screening and early detection programs on disease outcome, and com­pare the results of new treatments with the expected out­come without treatment.

There are some diseases for which there are no effec­tive treatment methods available, or the current treatment measures are effective only in certain people. In this case, the natural history of the disease can be used as a predictor of outcome. For example, the natural history of hepatitis C indicates that 80% of people who become infected with the virus fail to clear the virus and progress to chronic in­fection.²⁷ Information about the natural history of a dis­ease and the availability of effective treatment methods provides directions for preventive measures. In the case of hepatitis C, careful screening of blood donations and edu­cation of intravenous drug abusers can be used to prevent transfer of the virus. At the same time, scientists are striv­ing to develop a vaccine that will prevent infection in per­sons exposed to the virus. The development of vaccines to prevent the spread of infectious diseases such as polio and hepatitis B undoubtedly has been motivated by knowledge about the natural history of these diseases and the lack of effective intervention measures. With other diseases, such as breast cancer, early detection through use of breast self-examination and mammography increases the chances for a cure.

Prognosis refers to the probable outcome and prospect of recovery from a disease. It can be designated as chances for full recovery, possibility of complications, or antici­pated survival time. Prognosis often is presented in rela­tion to treatment options—that is, the expected outcomes or chances for survival with or without a certain type of treatment. The prognosis associated with a given type of treatment usually is presented along with the risk associ­ated with the treatment.