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Science > Biology > Health and DiseasesYou are Here
  • A major cause of ill health is ignorance, for example, ignorance about a balanced diet, or about the harmful effect of tobacco and alcohol. To achieve and maintains good health, basic knowledge about the body should be a part of education.
  • Good health of a person helps in improving his/her efficiency in a community the conditions necessary for good health are: Personal, domestic and community hygiene, Clean air, water and food, Balance diet, Exercise and relaxation and Proper community healthcare


  • A disease in any abnormal condition of the body or mind that causes discomfort or malfunction of a body organ. It can also be defined as a condition of the disrupted or deranged functioning of one or more organs or systems of the body caused by infection, defective diet or heredity. It can be a very mild ailment such as common cold or a sore throat or a serious condition such as cancer.

Causes of disease:

  • Every disease has a cause or a factor.

Intrinsic Factors:

  • Intrinsic factors are the factors present within the body. These factor cause malfunction of organs or glands, genetic disorders, hormonal imbalances, and allergies.
  • Disease caused by intrinsic factors is called organic disease or metabolic disease.
  • Example: diabetes occurs due to malfunctioning of the pancreas. Pancreas secret hormone called insulin in the required quantity which is essential for the metabolism of carbohydrates. The failure of the pancreas to produce insulin causes diabetes.
  • The disease caused due to intrinsic factors can be cured by proper medical treatment. Cardiac failure (heart attack), kidney failure, osteoporosis, myopia, cataract, sickle cell anemia, hemophilia, allergies, diabetes, and cretinism are all caused by intrinsic factors.

Extrinsic Factors:

  • Extrinsic factors are the factors present in the external environment i.e. outside the body, such as micro orgasms, pollutants, imbalanced diet, habit-forming substance (tobacco, alcohol, and drugs ) that may cause disease in the body.
  • Example: The cause of malaria, typhoid, tuberculosis, etc. are present outside the body, in air, water or any other vector (carrier).

Studying the Causes of Disease:

  • Every disease has a cause or a factor. Some causes are immediate while some causes are contributory. Immediate causes are directly responsible for the disease. The contributory causes increase the chances of getting infected. Most of the diseases have more than one contributory causes.
  • Example: In case of malaria, the bite by mosquitoes is immediate cause while the dirty surroundings in which mosquitoes breed is a contributory cause.
  • Let us take an example to understand the immediate cause and contributory causes o a disease. A person suffering from loose motion approaches a medical practitioner (Doctor). He told following symptoms to the medical practitioner-Loose, watery stools, abdominal cramps, abdominal pain, slight fever, blood in the stool, bloating, nausea and an urgent need to have a bowel movement. Medical practitioner analyses symptoms and concludes that it is the case of diarrhea.
  • The cause of loose motion is an infection. Thus the immediate cause is a viral infection (the infection caused by a virus)
  • Now next step is to find contributory causes. The first question for this is from where does the virus come? Generally, the source of the virus is the unclean drinking water. The medical practitioner will ask if any member of his family or his colleagues at the workplace are facing the same problems and reported similar symptoms. Here he is locating the place from where the virus has entered in patient’s body. If the person says that some of his colleagues are showing mild symptoms. Which means the source of the virus is drinking water at the workplace is the contributory cause. Now his colleagues are not showing severe symptoms as he does. It means his immune power is not working properly. It may due to malnutrition, improper diet. Malnutrition and improper diet may be due to less income and are contributory causes. Sometimes genetic constitution may be the contributory cause for the improper working of the immune system.
  • The treatment at next level is at the social level. Why are all people not getting clean drinking water? Why are some people suffering from malnutrition? Why are some people not getting enough income to get nutritional food?. Answers to these questions are also contributory causes. These problems should be addressed at the government level.

Disease-Causing Agents:

  • The substance or the force which causes a disease by its excess, deficiency or absence is called disease-causing agent.

Biological Agents or Pathogens:

  • These include viruses, rickettsias, bacteria, mycoplasma, fungi, protozoans, helminths, arthropods, and other organisms.

Nutrient Agents:

  • These include food components like minerals, carbohydrates, proteins, fats, vitamins, and water.

Chemical Agents:

  • Endogenous Chemical Agents: They are formed in the body due to metabolic activities. e.g. hormones, urea, and uric acid.
  • Exogenous Chemical Agents: They enter the body from outside by inhalation, ingestion or inoculation. They include pollutants (fumes, gases, dust, metal particles,  other particulate pollutants), allergens (pollen grains, and spores).

Physical Agents:

  • These include heat, cold, humidity, pressure radiation, electricity, and sound.

Mechanical Agents:

  • These include chronic friction and other mechanical forces which cause injuries, trauma, sprains, fracture, and dislocations.

Dietary and Excess of Substances:

  • These include hormones, enzymes, etc. The disease may be due to absence, insufficiency or excess of these factors.

Genetic Factors:

  • Some diseases are caused due to genetic disorders.

Underdevelopment of Organs:

  • The lack of a part or whole organ is the cause of disease.

Unknown Factors:

  • The agents for certain diseases like peptic ulcers, coronary heart diseases, and hypertension, are not fully known.

Classification of Diseases:

  • The diseases may be broadly classified into two types: congenital diseases and acquired diseases

Congenital Diseases:

  • The diseases that are present right from the birth are are Congenital diseases.
  • These diseases are caused by genetic disorders due to abnormal genes or abnormal number and structure of chromosomes.
  • They may be caused by a single gene mutation (alkaptonuria, phenylketonuria (PKU),  haemophilia, sickle cell anaemia, colour blindness etc.) or due to chromosomal aberrations. (Down’s syndrome, Klinefelter syndrome, Turner’s syndrome) or due to environmental factors (cleft palate, harelip)
  • The gene and chromosome induced congenital defects are transferred to the next generation due to heredity. Environmental induced defects are not transferred to next generation.
  • They are expressed in the form of errors in metabolism. (Examples: Phenylketonuria (PKU), diabetes Mellitus, haemophilia, sickle cell anaemia, etc.) or defective body development or mental retardation.

Acquired Diseases:

  • The diseases those develop after birth are acquired diseases.
  • The acquired diseases can be broadly divided into two classes: Communicable Or infectious diseases and Non- communicable or non- infection diseases.

Classification of Acquired Diseases:

Communicable or Infectious Diseases:

  • Diseases caused by a microorganism and transmitted from an infected person or animal to another person or animal are the communicable or infectious diseases.
  • Examples: common cold, influenza, measles, plague, pneumonia, typhoid, tuberculosis, malaria, amoebiasis, etc.
  • The microorganisms (or microbes) that cause diseases in human are called pathogens and the human, in this case, is called the host. They include bacteria, virus, protozoa, fungi, and worms. Disease causing microorganisms are also called germs. Microorganisms enter our body through the air we breathe, through the fluids we drink and through our skin.

Non Communicable Diseases:

  • These diseases remain confined to the person who develops them and do not spread to others. They are further divided as
  • Organic or Degenerative Diseases: They are due to malfunctioning of some of the important organs such as lungs, central nervous system, heart, blood vessels. e.g. Heart diseases like arteriosclerosis, the rheumatic heart are due to the abnormal working of some part of the heart. Epilepsy is due to abnormal pressure on regions of the brain.
  • Deficiency Diseases: They are produced by the diet deficiency of nutrients, minerals, carbohydrates, proteins, vitamins, and hormones. e.g. kwashiorkor, beri-beri, goitre, diabetes, etc.
  • Allergies and Inflammations: They are caused when the body, which has become hypersensitive to certain foreign substance, comes in contact with that substance. e.g. hay fever, sneezing due to pollens, etc. Other allergens are spores, dust, silk, nylon, asbestos, cotton, certain drugs, volatile substances in the atmosphere, etc.
  • Cancers: This is caused by uncontrolled growth of certain tissues in the body. It is abnormal, unwanted, uncontrolled tumour-like growth of undifferentiated cells in any part of the body.
  • Industrial Diseases: Caused due to pollution smoking
  • Mental Disorders: There is a change in thinking, emotion or behavior (or a combination of these).
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