The causes of disease 

Diseases can be caused by either environmental factors, genetic factors or a combination of the two. 

A. Environmental factors 

Environmental causes of disease are many and are classified into:

 1. Physical agents

 2. Chemicals

  3. Nutritional deficiencies & excesses

  4. Infections & infestations

  5. Immunological factors

  6. Psychogenic factors

1. Physical agents 

These include trauma, radiation, extremes of temperature, and electric power. These agents apply excess physical energy, in any form, to the body. 

 2. Chemicals 

With the use of an ever-increasing number of chemical agents such as drugs, in industrial processes, and at home, chemically induced injury has become very common.  Their effects vary: 

•  Some act in a general manner, for example cyanide is toxic to all cells. 

•  Others act locally at the site of application, for example strong acids and caustics.

Another group exhibits a predilection for certain organs, for example – the effect of paracetamol and alcohol on liver. Many toxic chemicals are metabolized in liver and excreted in kidney, as a result, these organs are susceptible to chemical injury. 

 3. Nutritional deficiencies and excesses 

Nutritional deficiencies may arise as a result of poor supply, interference with absorption, inefficient transport within the body, or defective utilization. It may take the form of deficiency either of major classes of food, usually protein and energy, or vitamins or elements essential for specific metabolic processes, e.g. iron for haemoglobin production. Often, the deficiencies are multiple and complex. 

On the other hand, dietary excess plays an important role in diseases in Western countries. Obesity has become increasingly common, with its attendant dangers of type 2 diabetes, high blood pressure and heart disease. 

 4. Infections and infestations 

Viruses, bacteria, fungi, protozoa, and metazoa all cause diseases. They may do so by causing cell destruction directly as in virus infections (for example poliomyelitis) or protozoal infections (for example malaria). However, in others the damage is done by toxins elaborated by the infecting agent as in diphtheria and tetanus. Like chemicals, they may have a general effect or they may show a predilection for certain tissues. 

5. Immunological factors 

The immune process is essential for protection against micro-organisms and parasites. However, the immune system can be abnormal which can lead to diseases. The abnormalities of the immune system include:

 A. Hypersensitivity reaction

This is exaggerated immune response to an antigen. For example, bronchial asthma can occur due to exaggerated immune response to the harmless pollen.

 B. Immunodeficiency

This is due to deficiency of a component of the immune system which leads to increased susceptibility to different diseases. An example is AIDS.

C. Autoimmunity

This is an abnormal (exaggerated) immune reaction against the self antigens of the host. Therefore, autoimmunity is a hypersensitivity reaction against the self antigens. For example, type 1 diabetes mellitus is caused by autoimmune destruction of the beta cells of the islets of Langerhans of the pancreas.

6. Psychogenic factors 

The mental stresses imposed by conditions of life, particularly in technologically advanced communities, are probably contributory factors in some groups of diseases. 

B. Genetic Factors 

References

Bezabeh ,M. ; Tesfaye,A.; Ergicho, B.; Erke, M.; Mengistu, S. and Bedane,A.; Desta, A.(2004). General Pathology. Jimma University, Gondar University Haramaya University, Dedub University.