Disease according to Merriam dictionary; is a condition of the living animal or plant body or of one of its parts that impairs normal functioning and is typically manifested by distinguishing signs and symptoms. Affection, ailment, disorder, fever, ill, infirmity, sickness etc. are also used as synonyms for the term disease. 


The levels of a disease in a population over a period of time and how individuals move from one level of disease to another in the population i.e. stages of disease are discussed below.

Here, Figure 1 shows the timeline for the development of a disease in an individual. Let us suppose an individual is healthy i.e., without disease, and at some point, biologic onset of a disease occurs. This could be from any means of disease transmission. The person is often unaware of the point in time when the disease begins. Many times the person might not show symptoms and just carry the disease and keep spreading. Sometimes later, symptoms develop and lead the patient to seek medical care. In certain situations, hospitalization may be required, either for diagnosis or for treatment, or sometimes for both. In any case, at some point a diagnosis is made and treatment is initiated. After treatment there might be one of the several outcomes such as:  the disease might have been cured, the diseases might have been controlled, and sometimes the person might suffer from disability and sometimes or many a times death.

In short one can say Stages of Diseases are EAPCD, Exposure, Acquisition, Progression, Complications and Death/Disability if not treated.

In another figure here that shows the development of disease in a population as reflected by the levels of illness and medical care. The biggest rectangle that is on the outermost represents the total population (Fig. 2A), and the smaller rectangle represents the smaller subset of sick people (Fig. 2B). As a person becomes ill, he moves within the sick group to those who seek care and to the subset of those who are hospitalized, from the outside rectangle to the progressively smaller rectangles in the diagram as shown by the curved arrows (Fig. 2C). As seen in Figure 2D, deaths occur in all of these rectangles, as shown by the small straight arrows, but the death rate is proportionately greater in groups with more severe illness such as those who are hospitalized.

What sources of data can be used to obtain information about the person’s illness? For the period of the illness that necessitates hospitalization, medical and hospital records are useful (see Fig.1B). If hospitalization is not required, physicians’ records may be the best source. If we want information about the illness even before medical care was sought, we may obtain this information from the patient using a questionnaire or an interview. If the patient cannot provide this information, we may obtain it from a family member or someone else who is familiar with the patient’s health status. Not shown in this figure are the records of health insurers, which at times can provide very useful information.

The source of data from which cases are identified clearly influences the rates that we calculate for expressing the frequency of disease. For example, hospital records will not include data about patients who obtained care only in physicians’ offices. Consequently, when we see rates for the frequency of occurrence of a certain disease, we must identify the sources of the cases and determine how the cases were identified. When we interpret the rates and compare them to rates reported in other populations and at other times, we must take into consideration the characteristics of the sources from which the data were obtained. Occurrence of disease can be measured using rates or proportions. Rates tell us how fast the disease is occurring in a population; proportions tell us what fraction of the population is affected.


  1. Merriam- Webster dictionary
  2. Epidemiology: Leon Gordis; Elsevier  5th Edition ;Chapter: 3 Pages (40-41).
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