1. Characterstic of disease

2. Incidence: is a measure of the probability of occurrence of a given medical condition in a population within a specified period of time. Although sometimes loosely expressed simply as the number of new cases during some time period

3. Incidence proportion (also known as cumulative incidence) is the number of new cases within a specified time period divided by the size of the population initially at risk.

4. For example, if a population initially contains 1,000 non-diseased persons and 28 develop a condition over two years of observation, the incidence proportion is 28 cases per 1,000 persons per two years, i.e. 2.8% / 2 Years.

5. incidence rate is the number of new cases per population at risk in a given time period .  In the same example as above, the incidence rate is 14 cases per 1000 person-years, because the incidence proportion (28 per 1,000) is divided by the number of years (two).

6. Incidence vs. prevalence
Incidence should not be confused with prevalence, which is the proportion of cases in the population at a given time rather than rate of occurrence of new cases. Thus, incidence conveys information about the risk of contracting the disease, whereas prevalence indicates how widespread the disease is.

7. Prevalence is the proportion of the total number of cases to the total population and is more a measure of the burden of the disease on society with no regard to time at risk or when subjects may have been exposed to a possible risk factor. Prevalence can also be measured with respect to a specific subgroup of a population.

8. Incidence is usually more useful than prevalence in understanding the disease etiology: for example, if the incidence rate of a disease in a population increases, then there is a risk factor that promotes the incidence.

9. Aetiology:
A branch of medical science concerned with the causes and origins of diseases. Where no etiology can be ascertained, the disorder is said to be idiopathic. There are two major classes of etiologic factors: genetic and acquired (infectious, nutritional, chemical, physical, etc).

10. Aetiology is sometimes a part of a chain of causation
Aetiology is sometimes a part of a chain of causation. An etiological agent of disease may require an independent co-factor, and be subject to a promoter (increases expression) to cause disease.

11. An example of all the above, which was recognized late, is that peptic ulcer disease may be induced by stress, requires the presence of acid secretion in the stomach, and has primary etiology in Helicobacter pylori infection.

12. Many chronic diseases of unknown cause may be studied in this framework to explain multiple epidemiological associations or risk factors which may or may not be causally related, and to seek the actual etiology.

13. Pathogenesis:
Pathogenesis means the mechanism through which the cause operates to produce the pathological and clinical manifestations. The pathogenetic mechanisms could take place in the latent or incubation period.

14. So pathogenesis  of a disease is the biological mechanism (or mechanisms) that lead to the diseased state. The term can also describe the origin and development of the disease, and whether it is acute, chronic, or recurrent.

15. Types of pathogenesis:
include microbial infection, inflammation, malignancy  and tissue breakdown. For example, bacterial pathogenesis is the mechanism by which bacteria cause infectious illness.
Most diseases are caused by multiple processes. For example, certain cancers arise from dysfunction of the immune system  (skin  tumors  and lymphoma after a renal transplant, which requires  (immunosuppression)

16. Morphologic changes 
The morphologic changes refer to the structural alterations in cells or tissues that occur following the pathogenetic mechanisms. The structural changes in the organ can be seen with the naked eye or they may only be seen under the microscope.

17. Those changes that can be seen with the naked eye are called gross morphologic changes & those that are seen under the microscope are called microscopic changes. Both the gross & the microscopic morphologic changes may only be seen in that disease, i.e. they may be specific to that disease.

18. Therefore, such morphologic changes can be used by the pathologist to identify (i.e. to diagnose) the disease. In addition, the morphologic changes will lead to functional alteration & to the clinical signs & symptoms of the disease.

19. Clinical features: Relating to a clinic, relating to the bedside of a patient, the course of his disease, or the observation and treatment of patient directly. The morphologic changes in the organ influence the normal function of the organ. By doing so, they determine the clinical features (symptoms and signs), course, and prognosis of the disease.

20. In summary, pathology studies:-
Etiology 􀃎 Pathogenesis 􀃎 Morphologic changes 􀃎 Clinical features & Prognosis of all diseases.