Concept of Disease

"A condition in which body health is impaired, a departure from state of health, an alteration of the human body interrupting the performance vital function". -Webster

"Disease is the maladjustment of human organism to the environment".- Ecological view

"A condition of the body or some part or organ of the body in which its functions are disrupted or deranged". - The oxford English Dictionary

WHO has defined health but not disease, because of the following limitations.

Disease has got a spectrum varying from subclinical state to severe illness.

Onset may be sudden (as in food poisoning) or insidious (as in leprosy).

• The diseased person may be apparently healthy but may be spreading to other (as carrier state).
• The same pathogen may cause more than one disease (eg: streptococci).
• The same disease may be caused by more than one organism (eg: diarrhea).
• The course of disease may be short or prolonged.
• The final outcome of disease is variable,i.e., recovery, disability or death.

While discussing the concept of disease, it is better to understand few terminologies related to disease, as stated by WHO as consequences of disease.

Disease → Impairment → Disability → Handic

• Impairment: Any loss or abnormality of psychological, physiological or anatomical structure or function. It may be visible or invisible, temporary or permanent, progressive or regressive.
• Disability: Any restriction or lack of ability (resulting from impairment) to perform an activity in the manner or within the range considered normal for human being.
• Handicap:  A disadvantage for a given individual, resulting from an impairment or disability that limits or prevents the fulfillment of a role that is normal (depending on age, sex and social, cultural factors) for that individual. Example: Disease: Accident, Impairment: Loss of foot, Disability: Cannot walk, Handicap: Unemployed

Epidemiological Triad/ Ecological Concept

The concept of disease causation is broad. The flaws of the germ hypothesis gave rise to the idea. According to this theory, disease arises when the equilibrium between the agent, the host, and the environment is disturbed. The interaction between the host, agent, and environment is known as the ecological or epidemiological trio.

• Agent Factors

A disease agent is defined as a substance, living or non living or a force, tangible or intangible, the excessive presence or relative lack of which initiates the disease process. The disease agents are broadly classified into the following groups.

• Physical Agents: Heat, cold, radiation, noise, atmospheric pressure, humidity etc.
• Chemical Agent: a)Endogenous (chemicals produced within the body urea, uric acids, bilirubin, ketones, calcium, oxalate etc.

Exogenous (agents arising outside human host) Dust, gas, fumes, metals, allergens etc.

• Biological Agents: For disease to be produced, a biological agent should have the following features
  • Pathogenicity (ability to produce illness).
  • Infectivity (ability to invade and multiply in host)
  • Virulence (ability to produce and fatality)
  • Viruses, rickettsiae, bacteriae, fung protozoa, helminthes, arthropods etc an examples of biological agents.
• Mechanical Agents: Friction, force, injury, sprain, accidents etc.
• Nutritional Agents: Nutrients like proteins, fats, carbohydrate vitamins, minerals and water; the excess or deficiency of which results in disease.
• Social Agents: Poverty, smoking, alcohol, drug abuse, social isolation, unhealthy lifestyles etc.
• Host Factors

Host refers to human or animal that comes in contact with the agent. These are the factors in the individual which determine the outcome of the interaction among three factors. These are as follows.

• Age: Certain diseases are peculiar in certain group. For example, measles and diphtheri among children, hypertension and diabetes among middle aged. ag
• Sex: Certain diseases like lung cancer and coronary heart diseases are common among men and rheumatoid arthritis, hyperthyroidism, diabetes, obesity are common among women. Disease of prostate occurs only among men and toxemia of pregnancy only among women.
• Ethnicity: Ankylostomiasis is less frequent and sickle cell anemia is more frequent among Negroes. Thalassemia is common Mediterranean region. people
• Occupation: This not only determines the income but also the health hazards arising out of the occupation. Eg: Pneumoconiosis (an occupational lung disease caused by the inhalation of dust, often in mines and from agriculture), accidents, lead poisoning.
• Literacy Level: Higher the literacy level, lower is the incidence of the disease. Lower the literacy level, higher is the incidence of the disease due to lack of knowledge.
• Income: This is the key factor determining the standard of living and influencing the development of the disease. Lower socioeconomic status predisposes for infectious disease and higher status for non communicable disease.
• Marital Status: Cancer of cervix is common among married women than unmarried women.
• Nutritional Status: Poor nutritional status makes a person mor vulnerable to infectious diseases.
• Genetic Factors: Certain diseases are determined by genetic factors like behavioral factors, chromosomal abnormalities, blood disorders etc.
• Immunity: The reaction of human host to infection depends upon his previous immunological status. Infection, immunization, individual having natural or acquired immunity is not easily susceptible to diseases.
• Culture and Customs: Certain customs and culture may have a positive or negative effect on health. Open field defecation favors transmission of diarrheal disease, worm infestation etc.

• Lifestyle Factors

Smoking, alcoholism, drug abuse, lack of exercise, multiple sexual partners etc favor the development of disease like lung cancer, liver disease, obesity, diabetes, heart disease, STDs etc. respectively.

• Environment Factors

All that which is external to individual, human host, living and non-living and with which s/he is in constant interaction is called environment. There are three types of environment.

Physical Environment:

Air, water, soil, housing, climate, geography, heat, light, noise, debris, radiation etc are included as non living and physical factors. Alteration in their environment may lead to disease. Excessive noise referred to as noise pollution may lead to deafness; air pollution may lead to various respiratory diseases.

Biological Environment

Viruses, other microbial agents, insects, rodents, animals and plants, some acting as disease producing agents, agents, reservoirs of infection, intermediate hosts and vectors.

Psychosocial Environment

Psychosocial factors, such as being exposed to stressful situations like death or divorce, desertion, job loss, the birth of a child with special needs, etc., cause feelings of anxiety, tension, anger, depression, and frustration, which increase the risk of diseases like hypertension, headache, duodenal ulcer, bronchial asthma, and mental illness, among other things. Worries and sadness frequently act as risk factors for crime, violence, suicide, murder, etc. Customs, beliefs, traditions, taboos, cooking methods, feeding and eating customs, etc. are examples of cultural practices that have an impact on community members' health.