Monday, 18 April 2016

CAUSES OF DISEASE

There are three factors which are responsible for the spread of a disease. These factors include agent, host and environment. These factors are known as epidemiological triad. If any one of these factors is missing then disease cannot develop. Agent, host, and environmental factors interrelate in a variety of complex ways to produce disease. Different diseases require different balances and interactions of these three components.
These three factors are described below:
1.    AGENT
Agent is the most important factor for the occurrence of disease. Generally, the agent must be present for disease to occur; however, presence of that agent alone is not always sufficient to cause disease. A variety of factors influence the occurrence of disease.
The disease agents include:
Biological agents: bacteria, viruses, fungi, protozoa, worms and insects.
Nutrient agents: carbohydrates, proteins, fats, vitamins, minerals and water. An excess or deficiency of these nutrients may cause nutritional diseases.
Physical agents: extremes of cold and heat, X-ray, y-rays, pressure, electricity etc.
Chemical agents: carbon monoxide, pesticides, fertilizers, fumes, dust, gases etc. may cause illness by inhalation, ingestion or direct contact.
Mechanical agents: motor vehicles; machinery etc. may cause injuries and fractures.
2.    HOST
Host is the person or population on which the agent acts. The host related factors includes age gender, ethnicity, socio-economic status, marital status, nutritional status, genetic, occupation, physiological or psychological state, preexisted disease and lifestyle characteristics.
Each of these factors will have an impact on the development of disease in the individual host. Man acts as a host to a number of pathogenic micro-organisms. These micro-organisms attack the host when the immunity (defense mechanism) is lost. 

The host factors are:
·   Age plays an important role in developing disease. Ex: Certain diseases like measles etc. are more common in childhood, cancer in middle age and tuberculosis in old age.
· Studies showed genetic factor played a role in the development of disease. Ex: sickle cell anemia, hemophilia, essential hypertension, Diabetes, etc are genetically transferred diseases.
·  Gender also plays a role in developing disease.   Ex: Females are more affected from cancer of various organs than males especially breast cancer. Cases of heart attacks are more in males than in females. Other illness due to gender includes Essential hypertension, diabetes, mental diseases etc.
·   Sickle cell anemia is more common in certain ethnic groups like People of African descent, including African-Americans.
·  Physiological and psychological state of host is also considered in disease occurrence. Ex: pregnancy, obesity, high blood pressure etc. 
   Stress is one of the important psychological factors in developing disease.
·  Pre-existence of diseases and immunocompetence are also key factors affecting susceptibility and resistance to diseases.
·  Nutritional factor such as Deficiency of proteins may cause kwashiorkor and other problems. Over-eating may lead to obesity and diabetes. Certain habits like smoking may cause lung cancer; open air defecation may cause soil and water pollution which ultimately causes various types of intestinal disorders.
3.     ENVIRONMENT
Environment plays a great role in the maintenance of health. A healthy environment is crucial for the health and well being of individuals and communities. Environment is the third concept inherent in the epidemiological triad in causing disease. It may be favorable to host or to agent. If the environment is favorable to agent it will cause disease. The interaction of host and environmental factors is very important in tracing the etiology of disease.
Negative environmental factors are threats to health, and controlling them is public environmental health. They include:
· Climatic and seasonal factors may determine whether it will be suitable to a particular disease or not e.g. malaria is more common in rainy season whereas common cold is more common in winter. Pollen and pollution in air leads to respiratory diseases or cancers
   Environmental disruptions like floods, droughts, storms, fires, earthquakes, volcanoes affect the health.

Sanitary conditions, provision of potable water supply, education and standard of living as well as biological environment e.g. animals, insects, rodents etc. are important factors in determining incidence of diseases.
· Release of noxious chemicals like mercury becomes poison. Create temporary, intense, life-threatening heat islands (e.g. urban heat waves exacerbated by climate change) results from nuclear, biological or chemical warfare etc.





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